Matrix (https://matrix.org) is a good alternative to both Slack and Discord. The most complete client implementation is Riot (https://riot.im/).

The protocol itself is federated, so you can communicate with other Matrix users from your self-hosted instance. There are also bridges to IRC, XMPP, even Slack..

Gitlab itself ships with Mattermost <https://mattermost.com/> :)

This doesn't look half bad... while slack's down, seems like a pretty optimal chance to try https://github.com/mattermost/mattermost-server

Atlassian, the folks behind bitbucket, jira, and (now) trello have a self hosted product: https://www.atlassian.com/software/hipchat

They also have a Slack alternative, called Stride.

And HipChat is horrible. Doesn’t even sync which multiple devices and their mobile app doesn’t support the iPhone X screen size which is a trivial update to make considering HipChat is used by some pretty massive customers. Code highlighting is still, (after years,) pretty bad.

Hipchat doesn't exactly have a stellar uptime record either.

Has anyone here used Stride? Reviews?

https://zulipchat.com/ has a topic/thread concept that is just awesome!

Already tried Movim (https://movim.eu/)? It's fully relying on XMPP.

I use gitter.im for some oss community chat. It's okay.

We're giving Mattermost a good try now!

Mattermost is actually integrated with GitLab - so yes - options do exist.

There’s stuff like Rocket Chat and I think Dropbox released one as OSS a while back resultant from an acquihire, but the name escapes me now.