From what I understand, crop top is simply a generic term for clothes that are shorter than their original length to show their navel.
It's a tag that's used not only for shirts, but also for tank tops, sweaters, and even jackets, right?
Idk it's meaningful to subdivide these tags like the ones on the BUR.
If crop top is being used to jackets while we have cropped jacket, then I guess it's either wrong or a implication cropped jacket -> crop top is missing. T-shirt implies shirt and we have decided that blouse should imply it too, but tank top doesn't. When I google "tank top", all I get are shirts, cropped or not, but in danbooru, it's being used for stuff like post #5423447, post #5422487 and post #5415895, I really wouldn't call them a shirt.
So if tank top can't imply shirt by that logic, crop top shouldn't either, because tank tops are a subtype of crop tops by our definition. Still, having *_crop_top tags is also unnecessary granularity...