Then again, look at the stacks you're looking at: they're all relatively new, and most of them are frontend, and thus more likely to jump on the new shiny. ##c on freenode has >1k users as I write this. Although that's probably also an outlier.

Personally, I'm hoping that Matrix becomes the new standard, if we ever get one beyond IRC. IRC is showing its age, and Matrix seems to be the most practical and levelheaded group doing work on the problem.

As for Slack, I'm pretty lukewarm on it, and kinda prefer IRC.