It's very much alive. Tens of thousands of people are on Freenode.
> I’m trying to hang out on Linux and open source channels on Freenode, but most of them are support related.
Try finding, or starting your own, channels that are little more specific. When someone asks for help participate and try to get the conversation going.
> I really miss the underground atmosphere of IRC with idling, homemade bots and what not. Does anyone here still hang out on IRC or Matrix and where?
I still hang out on IRC. There's still that "underground" atmosphere in some networks. You just have to look.
> Where should I find my digital social life? (PS - I’m in Australia and I don’t like Facebook, Reddit or Twitter.)
IRC channels and forums that match your interest.
Its still mostly me doing lunatic redneck hacker ranting about whatever comes to mind. We'd welcome more voices :)