I use Nerimity and Matrix (Nitro Chat) a lot (I actually own a Nerimity server or two, and that’s for my YouTube channels).
Rocket Chat is MIT licensed and self-hostable as far as I’m aware. Stoat is absolutely toxic, and I’m actually persona non-grata on the main Stoat instance (personal reasons I can answer via DM), Zulip is quite nice, despite the UI, and Quiet I’ll need to take a look at.