should i cut this person slack due to their disability?

No. Autism is not a reason to be a bigot. Maybe the “lashes out” could be explained by sensory sensitivity (depending on the scenario and what you mean by “lashing out”), but that doesn’t excuse bigotry.
Edit: also, what makes you say she is “mentally younger”? Autism and ADHD don’t stunt maturity in and of themselves
- There seems to be evidence that ADHD brains are a few years behind in development. I think I remember it being 3 years on average? Don't quote me, I have ADHD and my brain shouldn't be trusted with details. Anyway, that really shouldn't result in the kind of behaviour OP is describing though.
  
  I think I remember it being 3 years on average?
  No chance in hell it's linear, the gap would definitely change with age. But, as an adult with ADHD, I have certainly always felt a little younger than all of my peers... obviously anecdotal, but 🤷‍♂️
- No. Autism is not a reason to be a bigot.
  Autist here, completely agree.
  Many subtle, more context dependent social cues took me a lot longer than NTs, Allistic people, to figure out... though I excelled at school and have gone on to hold highly technical data analysis/reporting, db admin type jobs.
  Blatant bigotry is... not a very subtle or context dependent thing to understand.
  This fairly young 19F girl comes from a bigotted family that has completely or mostly normalized this kind of behavior, and has also infantalized her into believing Autism and ADHD are excuses for her poor behavior, as opposed to explanations.
  Right wing bigots tend to treat Autism (and really any mental disorder) as basically 'they're removed, just expect them to be shitty, and also I am a hero for raising a removed baby', as opposed to actually taking time to learn the ins and outs of how their minds operate differently, and learn together how to bridge that gap, with a bit of accomodation coming from both sides.
  This often results in infantilization of the kid, of just taking away their agency, instead of actually putting in the extra work to help them build up their agency and tweak or tune their worldview to be a bit more aligned with, or at least aware of, how much of the world doesn't operate by the rules that an Autistic person would default to.
  (Just go look at how RFK Jr apparently think we are literally pants shittingly stupid and will never pay taxes or go on a date... given the Kennedy family history of literally lobotomizing his own aunt I think it was, for her mental disorder... yeah not looking great for us NDs with this frat boy fail son with a brain worm where his brain should be as fucking Health Secretary.)
  Right wing idiot bigots are not very good at critical thinking, so... yeah, it makes sense that they also suck at teaching critical thinking.
  I have often seen this produce many additional behavioral problems in other younger Autistic people... because their idiot familes basically Munchausen-by-proxy their Autistic kids into beleiving they are far, far less capable generally than they actually are.
  In a sense, her family was probably bigoted toward her by treating her as a caricature of what Autism actually is during her most fundamental developmental years... needlessly stifling her mental development... so now she is doing the same and broadly being bigoted toward other people with other 'labels' that fit into other 'boxes'.
  This girl needs to learn to stop excusing her bigotrd shittiness by pointing at her mental conditions.
  There are plenty of people with Autism and/or ADHD who ... yes their minds work differently, but they aren't all raging bigots, thats on her.

Hi! I'm also a lady with au/ADHD.
Ella is a twat. Having a disability/handicap does excuse dehumanizing someone else. She can be hateful with her parents if she really wants, but don't tolerate that shit.
She's on a slippery slope for a larger part of society to start dehumanizing her based on her diagnosis/identity, too. Glass houses, I guess. 💅
On a slightly related note, some kid I went to school with constantly got away with touching girls inappropriately because he blamed it on his ADHD. He gleefully kept getting away with it. It was absolutely disgusting. He graduated and ended up going to prison a couple years later. Turns out, "it was my ADHD!" is not a viable defense in the real world! 🤡
- Reminds me of Rosanne when she tweeted racism and then blamed it on Xanax. Guess what she does now? Hint, it's not TV and it involves a red hat.
  
  Turns out, being a shitty person makes you a shitty person. Who knew?

Behavior like this is corrected by people letting you know that it's not okay. If you let it slide it will continue. I'm not saying you need to jump down her throat or whatever, but her parents have clearly failed to correct it (and probably encourage it). So your choice is to either accept it and defend her to everyone she alienates or to politely suggest that it's not okay and if she's not willing to change that you don't want to be around people who act like that. This shit only flies because no one has imposed consequences for this hateful behavior. I don't care how mentally developed she is, anyone can understand that hating others for who they are is bad.

Autism or not, this isn't an acceptable way to behave. You should be firm and tell her that she is wrong. I don't know about cutting contact but for sure that can be an option if she doesn't change. You don't need homophobic transphobic people in your life.
Not aiming this at you but: when did it become socially unacceptable to condemn / chastise people with ADHD / autism when they say or do unacceptable things? This only emboldens them to do worse things.
Also:
so this one girl, i’ll call her ella
Laughs in Spanish
- Perfecto! Ella se llama “Ella”

Being "mentally much younger" is not an excuse to be an asshole. I've personally known five and six year olds who understand quite well that some people love and marry people the same gender as themselves, and also accept being corrected on whether someone is "he," "she," or even "they."
Bigotry isn't natural, it's learned behavior you can accept and reinforce through your responses to her, attempt to correct, or simply judge her by and decide whether or not to continue involving her in your life.
(If Ella isn't capable of matching the mental age of a toddler, the help she needs is probably beyond your ability or responsibility to provide.)

Ask her if her parents jumped off a cliff if she'd do it too.
No, it didn't answer your question, nor is it helpful or useful advice. I just think it'd be funny and her parents absolutely used it on her at some point... it's a mandatory phrase all parents are legally required to say to their kids at least once. It's part of the contract you sign when the hospital gives you a baby.

Cutting someone slack doesn't mean letting them go on behaving badly, it means understanding they need help to behave better.

Is there a reason you have to interact with this person? It seems like if you're in a situation where her response comes with an LOL, your best course might be just to not engage. If you're in a position of responsibility with teaching her how to interact then gently repeating that respecting how someone would like to be addressed is probably warranted, even if it doesn't seem terribly effective the first (many) times.

start calling her a he or him. tell her you're under no obligation to refer to her as her preferred pronoun.
after a day or two ask her how it made her feel to have her request denied. then draw the point that what you just did is how she treats people, and they felt the same way she did.
if she demands you to stop, tell her "this is just the way the world is".
- Not a bad plan. What parents fail to teach the society must finish.

My brother in laws little sibling is pretty autistic and has a history of being semi-trans-not-understanding..? He saw someone who is a trans male and started HRT a few months ago (i.e. does not fully pass yet) and when he saw the trans man and was told that he was a man he said "but he looks like a girl..?"
for context my brother in law is ALSO trans and has been fully out and passing for like 5 years (beard n all), so he should've understood what it was like for 'trans man' to be like his brother.
He's done this with a nonbinary person too, but tbf most people don't understand how nonbinary works without autism.
What I'm trying to say here is that there's a difference between hate and not understanding. Sounds like the person you're talking about is being hateful, so have no shame in cutting contact.
Also if you don't feel comfortable with someone you don't need an excuse to yourself to justify leaving.

While I know it's not exactly the same situation, bear with me. There's a leadership book called What got you here won't get you there by Marshall Goldsmith.
It's focus on C suite people, their habits, and how their personality affects their career aspects, etc.
The reason I bring it up is because in pointing out characteristic flaws the author talks about how people say, well this is who I am, or I'm just bringing my whole self to work as reasoning for behaving the way they do. Then the author goes on to say how this is an excuse for the unwilling behavior to change as people don't want to change who they are as a person, but rebuts with "is changing this one aspect of how you behave going to drastically change you as a person?"
While I acknowledge that this person is young, I would say that pointing out that not saying anything is an option. She doesn't need to change who she is, just how she behaves and respecting others costs nothing. Additionally, if she is not willing to look at how her behavior hurts others then she is unwilling to mature. Being neurodivergent is not a justified excuse to be mean towards others. While it may be harder for her to understand, explaining that her words hurt people emotionally and asking her why she thinks hurting others is okay could be a good starting point.
It goes back to the old saying, if you don't have something nice to say, don't say anything.

I can't understand your situation but, I personally would be cutting contact regardless of disability. Nothing stated effects your decision making process, they are willingly having those values even if they may not understand the impact of their decision, I would rather stay far away.

Just tell her autism doesn't exist and that she'll never hold a job, go on a date, or play baseball.
/s

You're gonna see a whooole lot of "ASD here, kick her arse"
Add mine to the pile.

No. Being disabled isn't an excuse for bigotry.

Nah. Being disabled, old, minority or family does not mean you are a good person.

Eh, even with that she should still understand consequences. I'd give her long time outs. Next time she does say that you are offended, and need time away. Start with a couple of days. When she does it again make it a week no contract. Make sure she knows its because she was mean and you don't want to be around mean people. Hold firm during that time.
I don't like just saying one and done, give them a chance to change their ways. Even with autism that is informing them that they were offensive, and that there are consequences to that. It's their cross to bear, and I think that's being very generous in helping them learn that.

she's guilty, mental illness doesn't mean total inability, they can learn and they should have empathy
- I'm no psychologist but I'm pretty sure there's no disability that makes you a bigot.

I can't speak for you, but in my current position, I don't have the mental space for reeducating a person like this so I kinda have to let them go and fall off the cliff as to speak. Hopefully they'll find a way back but it's hard to change bigots.

She doesn't need slack for that. She needs firm redirection. If she's not able to take that, then cut contact.

I mean cut em some slack sure, but don't feel responsible for her actions. It's good that you realise it and as someone with ADHD it's immensely helpful when someone points out obvious things I'm doing wrong.
Executive mental functions thats seemingly normal for you could be impossible for them sometimes. Expect someone like that act matured in another 10 years, unless they fall off a 'cliff'.
Usually society would spit them out much earlier

you can cut anyone off for any reason.

Part of growing up is challenging the ideas you got from your parents and learning to treat people with the respect you'd like them to use with you. If she wants to continue being friends she needs to do this part of growing up.

I suggest you tell her that this is a problem, and that this is affecting your friendship. That if she does not try to educate herself, that you will have to reconsider the friendship.
Tell her that trans people have something called gender dysphoria. Reference Wikipedia and DSM. Prove that this is a real thing. Just like her autism. The best would probably be to just send her a couple of links with a short, but honest take on how this is a problem for you. Let her read it herself om her own time.
The last thing you should do is let the friendship end without talking to her. Its important to confront people, and give them a chance to change their mind. People deserve that.

The only thing I'm looking at is the age 19. A lot of people are stupid when they're 19, including her. It could be that she doesn't have enough life experience to question what she's been taught, so she defaults to what she was taught.
It is up to you to decide to be around her as she grows up and you find out who she will become.

I'd cut contact and leave her alone. Just let her be.

she also gets mad when i call a trans man “he” and says “SHE’S A GIRL EVEN THO SHE LOOKS LIKE A MAN LOL”
Wait so you do this or she does this? I’m assuming it’s her?
- "Says" implies "she says". "I says" sounds like some mafia speak.
  
  she also gets mad when i call a trans man “he”
  You see this part right? This is why I’m asking

I think you have to decide who you want to hang out with. Do you hang out with people a lot younger. If not why? Im guessing likely because they are immature, no?