First time hearing about Mattermost. Good thing I found this article
We replaced Slack with Mattermost for one of the teams - and guess what we don't miss Slack there. Threads, push notifications everything works fine and you get more features at least compared to the free version of Slack
So is the winning strategy here to pick anything but the top dogs in the game and hope they never make the big leagues and start behaving like shit? Mattermost just seems like another risky dependency