Slack was originally a side project built by a game dev company as a set of scripts on top of IRC. https://en.wikipedia.org/wiki/Slack_(software)#History

IRC really is good. It's reliable too, but it really fell behind the times and never pushed forward with additions to the spec and such. When the clients and libraries tapered off too it just felt like it froze.

IRCv3 aimed to do this, with mixed results.