Along with 4chan back in the day (less today) I'd add pre-porn-ban Tumblr (non-mainstream due to having porn). That was the other huge one and was a point of origin for a lot of today's gender-fluid stuff and the overlap it has with fandom, anime, and also liminal and fringe subcultures like furries. (I am not dissing or judging, just pointing out that it's a thing. I don't care what people do in their bedrooms or if they like to LARP as animals. You do you.)
Today I feel like a lot of it has retreated into closed groups, which makes it harder to keep an eye on it. Frustrating for someone who likes to watch it, but also might be a good thing since it might insulate it a little from the culture vultures.
Pre-Internet it was music-drug-party culture (those three always went together), which was full of weirdos that ended up setting trends. Gay culture was big too, especially when the latter was persecuted and kept at the fringe. For example a lot of techno, house, rave, and similar electronic music came out of gay black culture, which is a minority subculture among a minority. Hip hop came out of poor urban black culture, which developed turntablism and weird ways of "creatively misusing" cheap synths because they couldn't afford expensive ones. Blues came out of the poor South, mostly among blacks, and rock came out of that. Etc.
There's a very weird paradox at work here where persecution of a minority or a subculture empowers them and vice versa. I'm not saying persecution is good. Obviously it massively sucks to be on the receiving end of it. But it forces you into a liminal space that's more creative. By "persecution" I also mean just being viewed as incredibly weird or offensive and being soft-persecuted through ostracism and deplatforming. That applies to incels, oddball 4chan autists, etc., subcultures that definitely get either mocked or reviled by a lot of people for various reasons (some valid some not).