Yeah, I'd say it doesn't come from tumblr either? Tbh I haven't really seen it be so prominent in any fandom I've been in until now, and it all seemed to flood onto AO3 all at once. I do remember seeing some similar things waaay back in the 2000s on FF.net, but never so many all at once I don't think?
Re: is this a new trend, or older? Movie character/insert self or reader