Federated social app: Discord-style servers and chat with ActivityPub.
https://github.com/y4my4my4m/harmony
I don’t remember hearing of it till now, yet it claims a lot of desired features. Github page shows only one contributor, one fork, 44 stars; lot of commits in the last week, oldest 3 years ago; GNU Affero General Public Licence v3.0.
Found via https://chat.btfree.org/@ozoned/3850a821-fc6d-431f-b235-feed68605f6e
One nice thing about XMPP is that it can theoretically use transports to connect to everything and then some.
I have seen a Matrix/ActivityPub bridge in action, though.