Speaking of federated decentralised IM solutions. This is once I’ve come across before and looked interesting, trouble is I haven’t yet tried it:
The promise of XMPP was to deliver federated instant messaging: anyone would be able to spin up an IM server capable of exchanging messages with any other XMPP server in the world. Unfortunately, XMPP never delivered on this promise. Instant messengers are still a bunch of incompatible walled gardens, similar to what AoL of the late 1990s was to the open Internet.