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

That's a shame, I interviewed there once, decided not to take it but it was one of the few places I could have seen myself working at, they seemed like decent folks trying to build something worthwhile.