* Free
* Handles thousands of users seamlessly
* Archived via bot same as free Slack
* No noticeable performance hit for high user volumes
* Wide variety of interfaces and integration bots to choose from
* Widely deployed and trusted across tech communities
It's called IRC.
Edit: I can tell more people are encountering challenges using Slack these days. In the past, a suggestion to use IRC would have been downvoted to hell and back!
A lot of people seem hung up on the history, file sharing, or some other feature that Slack has, so I thought I'd take a moment to expand on my ideal setup and how they translate from Slack:
* Long-lived documentation and important team knowledge: Wiki pages (which I prefer even when using Slack and pins or history search are available). Full history of edits is visible and it's easy to access/search.
* Discussions involving a large number of stakeholders: Email. Threads and the async nature of the protocol make it well-suited to discussions when users aren't all online at the same time and the discussion is more thoughtful and isn't moving quickly. Slack threads suck replies aren't very visible and history scrolls quickly in active channels.
* Discussions involving outages, current tasks, and other immediate needs: IRC. It's real time, cross-platform, etc. History can be archived for later searching, but CHAT HISOTRY != DOCUMENTATION. Post-mortems and that sort of thing should end up in a wiki or similar.
* File sharing can be done via some other service with a link pasted into chat. One extra step to paste the link manually isn't a big deal.