I'm surprised Mattermost doesn't get more love generally, it's fully oss isn't it? https://github.com/mattermost/mattermost
Mattermost's license statements are confusing and contradictory.[1]
Anything “sovereign” should decouple the protocol from the client software IMO, which isn’t possible with Mattermost.
I think Mattermost lost a lot of instance admins' trust when they recently decided to update the server to limit access to old messages without good reason. On self-hosted instances!
https://github.com/mattermost/mattermost/issues/34271