I use Mattermost and don't miss slack except for mobile apps. They released an OSS iPhone app for it recently and the Android one is about to be released. Self

I use Mattermost and don't miss slack except for mobile apps. They released an OSS iPhone app for it recently and the Android one is about to be released. Self-hosting, unlimited history, unlimited team size. They have great deployment docs for various methods and it was pretty easy for me to containerize it for docker. It has a slack compatible webhook integration so many slack integrations will "just work" with mattermost, if the integration will allow you to specify your own url.