Someone else pointed out Cinny, which is... Pretty close to a discord replacement on Matrix, actually. Also I know Stoat mentioned a few federation options that they'd use if they ever do federate (not on the roadmap), with XMPP being the one it seemed like they'd favor.

Also ngl it doesn't even seem like Matrix would be a good slack replacement, unless the UI you're using makes it more slack-like

Re: Discord section, yeah... I liked discord at the start. It was mostly for voice calls, with a decent chat interface (based on IRC's). Then it started getting used wildly inappropriately, which annoyed me, and then all of the horrific privacy stuff came up and it's just... Ah. Whelp. This ship's sinking fast.