There are still plenty of posts from before alternate costume was changed to exclude official costumes.
Correction, alternate costume for all intents and purposes always excluded official costumes, "characters wearing costumes other than the ones they are typically depicted in" suggests non-canon costumes (i.e. forum #16242 and forum #41710; though for some, it only suggested non-officially-named/tagged costumes, see topic #1316 and topic #3699; and even then you had cases like topic #2487 which just tags it on official costumes). This is further compounded by enmaided implying the tag (forum #39492). It's only since 2019 that, in order to resolve the issue of folks tagging canon stuff that official alternate costume was created (topic #15975, which I then brought up in topic #17206), though a tag like it had been proposed since late 2010 (topic #7217). You could say that we've only resolved to explicitly stating that they aren't included since 2020.