I know there is revolt as a FOSS alternative to discord but it's not federated.
Matrix and clients for it like Element have always been my go to for federated chat like discord/teams/mattersmost. The main missing feature is voice channels imho.
The lack of voice channels is huge for something that looks like it is trying to be a discord alternative. I really can’t believe it still isn’t really implemented.
Honestly though even setting aside the lack of voice channels there is a bigger issue. Getting people to switch to something like matrix. I have so much trouble trying to get my group to switch to ANYTHING new. Tried to get them to switch from iMessage to telegram or signal. They tried it for a day and then went back. Getting them to switch from razer comms to discord was even a removed back in the day. People just don’t like trying new things.
Sorry for the rant I just find this endlessly frustrating. Lol.