YouTube has been cracking down on alternative frontends.
Vanilla Invidious currently doesn't work well,
so most hosters paused and/or gave up.
Fijxu runs a fork of Invidious,
with their own modifications implemented to circumvent the blocks,
here's the source code if you're interested: https://git.nadeko.net/Fijxu/invidious
I've got big respect for Fijxu,
he's been doing a very good job of keeping Invidious alive + fighting against the YouTube crackdown lately, basically all on his own.