One thing I’ve noticed - as a general rule - is that if the show/book/game/etc postdates ao3′s rise to prominence/creation, it tends to not have dedicated specialist archives of some kind, the way there were (and are still, tbf) tons of dedicated Harry potter-fic focused websites (or that old fandom friend you may admit you go to, Adultfanfiction), some to HP in general, some to specific pairings (offhand I can think of several that - last I checked about a year ago, were still active and getting fics added) or the few remaining actively updated Buffverse fic websites (Elysian fields - I think that’s the name - is a Spuffy one, for example).
Red Versus blue had one a few years back, no idea if it is still there or still being actively updated. And I can think of others (but RvB, while still ongoing, again predates the rise of Ao3)
By contrast, the MCU doesn’t seem (AFAIK) to have a lot of dedicated, high-traffic websites.
The reason, I imagine, is that as people were driven off FFN or LJ by restrictive rules (and that’s assuming the media doesn’t predate both entirely or partially), sometimes sloppilly applied, before ao3, making specialist websites was their only option.
But now if someone wants to see fandom content that isn’t allowed on other sites, Ao3, with their policy of maximum inclusion of works, is much more likely to be the place people go to.