My only issue with IRC as the alternative to Slack is that its really user hostile. Clients largely look like some 90s programmer’s idea of a chat tool, and as discussed, persistence and sharing files and things are extensions or hacks on top of the protocol. Without addressing these, IRC is going to remain the ‘underground’ tool it is at the moment, with little support from enterprise tools and other integration providers.
Personally, I’m a big fan of MatterMost[0] as its also open source but provides a much more Slack-like experience which new users will be immediately familiar with. You can host it yourself and it works with lots of existing integrations.
[0] - https://mattermost.com/