Let's have some climatic music to accompany us in this thread.
It takes no genius to realize that the first five games are of a different era than the rest and their appearance is different. The few things binding them together with the newer creations are: ZUN, the game type and a handful of characters which appear in either generations; though they still have separate tags to either "forms" (hakurei_reimu_(classic); kirisame_marisa_(classic); alice_margatroid_(young) and kazami_yuuka_(classic)). I believe that it'll be of best interest, to also have a way to browse only the "old" content all grouped up, meaning that it'll cover all characters from Highly_Responsive_to_Prayers, Story_of_Eastern_Wonderland, Phantasmagoria_of_Dim.Dream, Lotus_Land_Story and Mystic_Square and the four chartags mentioned earlier. Estimated tag size: ~4000.
There is a pc98 tag, but I'd rather leave it for other pc98 creations (I don't believe that those will happen anytime soon, if ever) or images imitating pc98 appearance (pixel_art, much) OR the actual pc98 presence on images (see the brackets for creations).
Pixiv has an "old touhou" (pixpedia article) tag of some sort, but I do believe that touhou_(pc98) is the best to go with. All in all, pc98 is what makes them so unpopular/unknown/unreachable (okay, much less canon data does too). I also volunteer to spread the tag on my own, once I get to know whether it's wanted by someone more than myself. I'll give it a few days(3-5) to process in here, unless I'll be allowed to do that earlier. If no really opposing opinion occurs, I'll go ahead and do it after those few days pass (wiki creation included, all help in composing one will be appreciated).
Updated by Toks