suc provides Slack, Mattermost, etc.’s core features:

Real-time, rich-text chat,
File sharing,
Fine-grained access control,
Straightforward automation and integration with other tools,
Data encryption in transit
and optionally at rest,
state-of-the-art user authentication.

This paper shows how suc implements those features. suc stays small by leveraging the consistent and composable primitives offered by modern UNIX implementations