Provence, I think you rely too much on naming similarity when you make these proposals instead of how the tags are defined and the visual populations that make them up.
You used post #2322204 as a clear cut example of thigh boot, except if you covered up the foot portion you'd have no actual means of identifying it between a thigh boot or thigh stocking. Doing a random search of thigh boots and drawing up 100 images, I easily found 33 (so 33%) of the images had no real way of differentiating themselves from a stocking unless you could see the foot portion of it.
On the other hand doing a similar search with knee boot didn't yield a single result that could be confused with a knee sock. The reason for this though is pretty simple, and that's because being a much shorter object it is harder to not include the portion that identifies them as a boot. It's much more likely the entire item all together will get cutout of the image than leave an unidentifiable upper portion. There is also the fact that shorter boots are just commonly depicted as either bulkier, with a upper rim that's obviously not used with a sock, or readily depicted with a sock or stocking coming out of it. It's these differences that make it a different case than thighhighs.
It might be easier and better to simply reverse the knee_socks -> kneehighs alias, so this naming argument can't be reused.