shirtless is currently mistagged to the point of uselessness. I estimate one in five posts are correctly tagged, if that. The vast majority of posts are of topless males, largely tagged by users who genuinely don't know the topless tag exists (as I've confirmed via PM). shirtless solo -bra -topwear should produce next to no results, but instead it produces over 3000. I've been chipping away at tag cleanup, but at ~6800 posts, I could spend years at that and not finish.
This isn't going to change without a concerted, multiple-person cleanup effort. If we don't have the will and manpower for that, I think we need to seriously consider aliasing shirtless to topless. "Shirtless" is a valid clothedness category if it's consistently tagged correctly, but the fact is that it isn't, and currently it's doing more harm than good. All it really does at this point is split topless content across two tags.