In which sense?
I boycott both tools, but there are many respectable communities relying on them (unfortunately).
Indexing is still a problem for social servers as well, but unfortunately there are no alternatives. The only current solution is to use bots to upload archives of public channels to say the Internet Archive.
Getting someone on the g0v slack is as easy as giving them a link. The entire experience is straightforward from there.
Maybe one of the federated softwares is to that point and I don't realize it, I deploy a lot of federated software but I haven't done a chat application in a long time.
I think this just cannot work if this is called alternatives. GauntletWizard wanted to specifically point this out: it's a mistake to think that the core functionality of slack and discord is just chat rooms & messaging, their mechanisms that implies terrible privacy are part of the core, and you can't expect users to dissociate messaging from the rest and call it a replacement.
+1 for having «our thing», but promoting it by bogus comparison or replacement is setting it for failure.
Chats/voip when self hosted are more dangerous to its users. I know it from experience.
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.