One of the biggest channels ironically is one for an IRC client called "the lounge". Which is a very nice web based client with bouncer build in. Though I might be biased as I did contribute to the project years ago.
I was a dedicated IRC user for a long time, even going as far as developing this monstrosity: https://github.com/creesch/discordIRCd
At some point I didn't see the point anymore, as it clearly was just old guard sticking around with no one new joining.
It is a shame that discord as a closed platform is the de facto standard though. I am not sure things could have gone differently. Considering how glacial IRC development was at the time and the convenience and features Discord offers over alternatives like Matrix. I know that Matrix these days has a lot of the same features but certainly is not there as far as convenience goes.