Name: Mattermost
Category: Team chat (#1065)
URL: https://mattermost.com/
Mattermost is an open-source, self-hostable online chat service. It is designed as an internal chat for organisations and companies, and mostly markets itself as an alternative to Slack.[6][7]
Warning: Mattermost doesn't yet have E2EE.
Apart from lacking E2EE the community edition comes without any form of access control / policy rights. Any user can archive a channel (which then is gone from the channel list and excluded from search results) and only admins can recover it. Also any user can remove other users from channels and change channel names and topics. (https://github.com/mattermost/mattermost-server/issues/6320)
Thanks for the note, I don't think Mattermost can be listed due to that restriction.
Looks like they started fixing the archived channel stuff, so things might not be as bad anymore.
Most helpful comment
Thanks for the note, I don't think Mattermost can be listed due to that restriction.