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Revolt became Stoat and is not in Fdroid anymore.
(stoat.chat)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
Matrix space | forum | IRC
It very much depends on what you are looking for.
Matrix is pretty good, but hosting synapse (Matrix server) is resource consuming. XMPP is very nice I think but I have not tried voice there. Nextcloud Talk requires to have everyone registered on the Nextcloud instance and also needs somewhat beefy HW. Mattermost seems to be professional level stuff, did not try. I don't really know about other alternatives, that's why I am waiting for a proper voice chat in Stoat so that I can test it.
So far, Stoat seems to be THE alternative to discord. For gaming atm I'm still using discord (because I would be the one moving everyone, and I won't do it with a proper alternative. For gaming that is. Discord is one of the last proprietary chats I use.)
@Alfredolin Matrix has metadata leaks tho, and it isn't perfect. You could try Mattermost, I have done so yet. The thing I'm doing is just making groups in Signal or even just sticking to plain old Discord. It's kind of like YouTube, where it's hard to find an actually good replacement.
My threat model does not require me to have something better than a selfhosted Matrix server. The point is leaving capitalist leechservices and/or US-based services.