I did about two years on Soylent no overt issues. It made eating simple, and I didn't gain any weight, but didn't lose any weight either. Guts will not atrophy
Agreed on the pooping reduction.
I wouldn't recommend Soylent to anyone now, not because it is bad (it's better then the Standard American Diet), but because I've found better ways to manage food and health (and even less pooping heh)
ohh, after the pandemic kicked off I stopped doing Soylent (on which i developed high blood pressure, heh), and started a ketogenic protocol. Since then I've moved over to a zero carbohydrate protocol (carnivore).
Carnivore is a super-clean version of keto, very easy to follow, hard to make mistakes on, I've seen great results. I poop about once every two days - and its never urgent, I never fart. If your trying to minimize pooping you can't do much better.