IRC is a good substitute for being in the same room, but you still need more formal discussions recorded in issues and pull requests (the recorded part is quite important if you project is around for more than a few weeks).
Well now that you mention it, in the communities I care about, IRC was replaced by Gitter (https://gitter.im/).
Which is also linked to GitHub accounts and repositories. Ironically, Gitter was bought by GitLab and open sourced, but they haven't changed the implementation to allow for chat rooms linked to GitLab repos. At the glacial pace it is evolving, it wouldn't be unfair to say that Gitter is probably relying solely on OSS contributions at this point.
While gitter generally is a good idea I have yet to land in a active channel. The partly abondened IRC channels often still show a lot more activity even when they link to their gitter in their meta.
Not my experience, it depends on the community you're talking about I guess. Here's some channels I'm on, all being so active that I have a hard time keeping up:
I remember back in the day IRC was the place to be for Debian. Ditto for when I was doing stuff with Perl.