See the comment above. If you're using Slack to avoid managing infrastructure, there are plenty of great (read: open source) alternatives.

Even if IRC isn't for you, there's XMPP, Mattermost [0], Let's Chat [1], and Zulip [2].

[0]: http://www.mattermost.org/

[1]: https://sdelements.github.io/lets-chat/

[2]: https://github.com/zulip/zulip