I use mattermost as a free, open source Slack replacement. I like it a lot. I also use authentik as my SSO provider for my #homelab / #selfhosted stuff.
If you don't pay for the enterprise version of Mattermost, you don't get the SSO features. You can fake that by using their free GitLab integration. But you also don't get the capability to switch users from one form of authentication (email/password) to SSO after you set it up.
This is pretty obscure, and it involves a bunch of raw Postgres queries. But I figured it out and wrote a blog on converting Mattermost users from email/password to SSO
Mattermost Converting Users from Email to Gitlab authentication | Paco Hope
If you start your Mattermost Team Edition using email and password for users, but then you change your mind and switch to GitLab authentication, it’s not at all obvious or easy how to convert the existing users.blog.paco.to
