I would argue that IRC absolutely is already super fringe. People have memories of it, but I'm betting active use of IRC today is limited to very tech people.
But it was already kinda fringe, even back in the late 90's. People would talk on AIM or AOL chat, but it was rarer to find IRC users among the hoi polloi.
IRC was home to a lot of gaming communities, it was basically Discord before that existed. There used to be live commentary in channels (Twitch replaced this).
I don’t believe any gamers use it today but fringe-tech communities still use it, I think that’s what the parent is referring to.
Yes, their chat is still compatible with IRC, albeit with lots of extensions (which you can opt into using IRCv3 capabilitiy negotiation). More recently they've been adding UI elements on the main site that are web API powered instead of chat though (eg. polls, sending bits).
True but in the early 90s there was nothing else yet. But back then the whole internet was totally fringe of course.
Still, MIRC (as a client) was a common thing for all computer geeks until 2005-2010 or so in my circles. After that people started moving to whatsapp and the like. This includes people in the hacker/maker and game/lanparty community so very techy and niche but there it was still big back then.