If you don't want to use Slack for open source projects because Slack is in fact pretty bad at those workloads, fine. I agree! I think Slack does a pretty bad job at any group application where most of the people communicating don't already know each other.
Where you go off the rails is in suggesting that IRC is competitive with Slack. IRC is awful. It's a medium dominated by 7-bit clients with tiny message limits that provides virtually no useful metadata and no works-by-default history or search --- both things that virtually every open source IRC support channel b a d l y needs.
IRC does exactly one thing better than Slack: it's easier to join a new IRC channel than it is to join a new Slack project. The rest of IRC's "features" are red herrings, many of them more harmful than helpful.
Surely by now someone has built a Slack-like, perhaps with decent IRC support, that big open source projects can champion as a Slack- and IRC alternative.
It will be much easier to keep open source projects from trying to fit their square pegs into Slack's round aperture when people give up on promoting IRC.