Oh man, seeing folks suggest it as a Discord alternative always had me uninterested, because I don't even use Discord and it just seemed like yet-another-standard.
Now I'm reading this really technical title for a talk which mentions XMPP and I'm instantly sold.

Well, to be honest, "Movim" also sounded like a VC-funded startup. Looks like it's a bus-factor-of-1 open-source project instead, which I have significantly more trust in.