Freenode has about 85,000 people logged in right now and there's over 50,000 channels.
I'm in a bunch of tech related channels and there's hundreds (or 1,000+) people in most of them. It's still active and has been for 15+ years. Freenode is just 1 IRC network of many too but it's where a lot of tech channels are set up.
IRC is awesome. You can use it from a terminal (or get a GUI client that uses < 20MB of RAM), it opens instantly, you can connect to a bunch of channels and networks automatically (with password protection on your nick), typing feels instant and you get infinite log history. Doesn't get much better in my opinion.