I think Matrix is a good upcoming contender. The available clients are not really mature yet though. Element is excessively heavyweight and bloated, being Electron based. Other clients all seem to miss some essential feature or other. And there's no good archiving solution yet.
And Element are consistently just not working in various ways, different every time. For example, recently I tried to join a room address from another server and was told "can't join that because a server address wasn't specified" but another address on the same server says "room not found" so it clearly did know the server address.
And there's the whole thing where you have to always be signed into at least one device at a time. If there's ever a time when you're not signed into any device, you have to reset your encryption key which means losing access to all old messages plus sending a notification to everyone you're in contact with. And I've had times when the reset button also just didn't work, throwing some error, so I was just locked out.
Matrix has/had a great archiver at https://github.com/matrix-org/matrix-viewer - but it fell foul of the bizarre failure mode that folks who had explicitly set their rooms to be archivable had subsequently forgotten that they had, and then went absolutely berserk when they discovered their rooms having been archived: https://matrix.org/blog/2023/07/what-happened-with-the-archi.... This was one of the reasons given for libera to turn off the libera<->matrix irc bridge.
At the time we had much bigger problems to deal with, so we shelved it and have yet to get back to it.
