We have this rule established for the Touhou game tags:
NOTE: Due to most characters appearing in more than one game, only the "Touhou" tag should be used unless a specific game is directly referenced by the content of the post. All of the characters in a picture having debuted in the same game is not sufficient to constitute a direct reference. Depicting the full cast of a game (with or without the protagonists) will usually count as a direct reference, so long as no other characters are present.
This effectively allows someone to search for "primary" fanart (art referencing the games specifically) over "secondary" fanart (art based off of fanon), something that is useful for a portion of our userbase. While there is nothing explicitly prohibiting people from tagging game-specific costumes with their respective games, and some people do choose to do so (e.g. many posts in the perfect cherry blossom tag), imposing it as a standard would require a lot of gardening with little to negative payoff. For instance, Yakumo Yukari has a drastically different outfit in Scarlet Weather Rhapsody (see post #2707217), as opposed to the standard one in her other appearances (post #2459959). We could go through and manually retag all of those images with the game they appear in, but what would that accomplish other than flooding the tag and diluting relevant posts (e.g. post #1810937, post #921460)?
If we had these tagging standards when Phantasmagoria of Flower View came out, it might have made sense at the time to tag all (9) posts with the game's tag - it's a joke from the manual, after all. Now we can clearly see why that's a bad idea: the (9) joke was adopted by the fandom at large and taken far out its original context, and now (9) posts outnumber tagged PoFV posts nearly ten to one. I can see the same process happening with tanned Cirno: already, we're getting posts that are complete fanon, like post #2718088. It's best to be proactive and change the tagging now, instead of opening this discussion again after a few years have passed and there are a thousand more posts to garden.