[ale42]

We use self-hosted Mattermost (team version, i.e. without limits but no enterprise features like LDAP). Fine for a small team (around 40 active users here) where you can script account actions via the API, probably not fine when users become a lot more, or you might need access to the compliance functions for audit purposes, etc.

For us the free version of Slack was insufficient, the commercial one too expensive, and anyway, given that it's a cloud-based system, it's not compliant with our internal rules for confidential information (unless we can get some specific agreement with them). On the side, there is a bit too much analytics/telemetry in the Slack client.