while this site is definitely old, I think it's fair to call it "nostalgia" because it was already retro at the time it was created in... 2004-05?

discussion forums were a '80s and '90s thing, by the early '00s most English-speaking internet users had moved onto multimedia-centric and ego-stroking social networking sites like myspace and livejournal. 2channel was one of the very last pure discussion forums to arise (somewhat) organically, and that happened mainly because the Japanese internet as a whole resists innovation

if you call 4-ch "a relic of the old internet" you might also have to call, like, reddit "a relic of the old internet". unless the distinguishing factor is literally never getting any significant software updates