Any client/server based app for a company-controlled replacementt The server manages rights and auth.

Anything that accept OIDC or is federated for the second.

Matrix doesn't totally fit the bill yet, but zulip does, so that's what you should install.

But the one that surprisingly seems to be the best is probably email. We don't have the proper MUAs yet, but deltachat (https://delta.chat) shows that there is no reason for us to be limited to decades-old paradigms and should reinvent ourselves.

What is a MUA? All results consider it "makeup artist."

Thanks. Offtopic, but this is the second time in a week the person who answered my question gets downvoted (if that's what the reduction in contrast of your post indicates). I'm not sure why that's happening.