I would suggest running your own instance of Mattermost. Depending on your userbase, IRC on a channel on an existing public network (OFTC or Freenode) might work, although Freenode has been known to engage in censorship pretty heavily sometimes.
Lots of people, including Mozilla, seem to like and use Matrix, as implemented by Synapse.
The bottom of my linked post (keep scrolling) has specific recommendations for alternatives.
Cool thanks
If you fire up a VPS somewhere and install CapRover (pretty easy for even someone virtually nontechnical), I recently contributed a config for Mattermost to the CapRover "one click apps" repository, which does just what it says on the tin: a single click to deploy your own Mattermost instance. (My config even turns off the on-by-default Mattermost server spyware, so you're private-by-default.)
https://github.com/caprover/one-click-apps/blob/master/public/v2/apps/mattermost.json
Drop me a line if you need help setting it up.