According to the cat_tail wiki,

• This tag is generally not applicable to anthro characters.

Why is this? If I remembered seeing an image of an anthro feline with a canine_penis but cat_tail I would think feline anthro canine_penis cat_tail would be a legitimate search, but if it's being purged from anthro posts, that wouldn't work.

Probably because cat tails on cat characters is the assumed default, so cat_tail would be applied to non-cat and possibly kemonomimi characters wearing cat tails. At least that's my guess.

From context, cat_tail is specifically an animal_humanoid characteristic. As you say, this leaves a corner case of anthropomorphic characters with atypical tails, which appears to be extremely uncommon but should probably have a solution worked out in case it gets popular.

That specific line's been there since the cat_tail wiki page was created, which is to say that the whole wiki page probably needs to be rewritten to conform with the current language.