Several years ago I put up an XMPP server (Prosody) to chat with some friends in a private group, and it's still going strong. Years ago we used to have a mailing-list to keep in touch, but that eventually went quiet. I had run an XMPP server ~10 years ago (ejabberd), but that was before mobile chat utterly displaced everything else, it never took hold among my friends, and I stopped using it when Google Talk stopped federating, cutting me off from most of the people I had been occassionally chatting with.

The modern chat experience is all about the clients, and the mobile clients in particular, especially push notification support and seamless setup, so direct comparisons with XMPP to Matrix, et al, kinda misses the point, IMO. Conversations is a really amazing Android client. Our one iPhone member is content with Modal (I use the desktop version sometimes, but it's clearly designed for the iPhone). A new member uses, I think, Gajim on Windows; they don't want the distraction of chatting on their phone.

I host a bunch of other services on OpenBSD, all using the integrated base daemons--httpd, OpenSMTPD, NSD, etc--that sandbox themselves. I was hesitant to run a daemon like Prosody that didn't integrate OpenBSD security features, so I wrote my own module (mod_unveil) that uses pledge and unveil to sandbox Prosody: https://github.com/wahern/prosody-openbsd Most Prosody users host on Linux, and many of them seem to use Docker containers, which presumably offers some isolation, but I can't really speak to it.

The only non-private, large group chat I've joined recently has been the Prosody support MUC. I'm not a chat power user, but it seems to work just as well as any large chatroom. I've was content with ntalk and IRC back in the day, so I don't really get all the chat protocol bike shedding. In any event, I expect XMPP momentum (such as it is) to outlast all the interest in Discord, Matrix, etc.