I don't think thigh boots -> thighhighs is a good implication and this thighhighs under boots tag is a good example why. It exists mainly to work around this implication, because searching for boots + thighhighs doesn't work. Any time we have to invent new tags to work around our own implications, it's a sign of a bad implication.
It also creates really weird scenarios where, for example, knee boots are considered footwear, but make them a little bit longer and suddenly they're considered legwear and grouped in with socks and pantyhose. By this logic, knee boots should imply kneehighs because they're both knee-length.
To put it another way: if you really can't tell whether something is thigh boots or regular thighhighs, then you wouldn't tag it thigh boots to being with. By tagging something thigh boots, you're saying you really can tell they're boots and not just stockings.
It's like treating panties and bikinis the same because sometimes they look indistinguishable. That may be true, but people still wouldn't want them to be grouped together.