For those who may have forgotten, Mattermost is quite good these days: https://mattermost.com/

Ha, I’ve had a Mattermost instance for years until they handicapped the most recent version by limiting the number of messages on the self hosted version.

I ended up building my own alternative and was going to OSS it but like… there’s already a bunch out there.

Anyway, Mattermost might not be the choice these days. With that stunt I was annoyed enough to spend a weekend to replace what they were to me.