A sufficiently advanced IRC client may be able to catch up to Slack in some of its essential features, such as formatted code sharing, image sharing (for screenshots and other things), document sharing, comments on uploads. etc.

But at that point, why not just use Slack or Mattermost.

And IRC is not even close to an adequate solution for what Slack does when it comes to non-devs. If you only ever talk to people who code, then perhaps you can work around IRC's deficiencies. But by using Slack, the conversation can be opened up to all sorts of non-technical people, and it enables them to communicate effectively so much better than IRC ever could. These non-technical people's contributions are essential to the functioning of almost all serious companies, meaning that for most teams, IRC will not work and Slack will.

IRC is a telegram network operating on Morse code compared to Slack's cellular network.