Matrix with Element (Riot) as the front-end is pretty close. It does what slack does, it's just not very good. XMPP is arguable. It can be a Slack alternative, if you stitch enough other servers on top of it. Personally, I don't think XMPP will ever be more than chat, but some of its adherents believe differently.

Mattermost is certainly not what I meant. That's just trading one Slack for another.

This thread is about a Slack outage, which you have no control over. Mattermost and similar software is self-hosted, which of course doesn't mean you're getting 100% uptime, but you have (more) control over it.

In practice self hosted usually translates to more downtime and slower performance when it works. Unless your org has more expertise running a chat service than Microsoft or slack, your self hosted alternative is always going to suck more.

Did you try self-hosting and it lead to more downtime and slower performance?

From my experience, when I self-host stuff it's a lot faster (more server resources) and never had any downtime (server doesn't simply go down for no reason).

I haven't tried it personally. But my employer hosts an on-prem Github instance and it is just terrible. So many downtimes, long times before anybody gets around to repair it, general performance issues, maintenance windows for upgrades, etc. Just a huge pain. I've seen this sort of problem with the old Exchange on-prem services too.