Browsing for 3d will give you horror. Page one is 33 of 36. And of the other 3 one had a subgenre of horror and one tagged gore.
Not that other tags are any better. Sure that over 90% popular games on itch are all horror exclusive? That somehow does not sound that right. Should it not diversify after entering a tag?
Anyways, if there is tags free, one can use it, but for searching it is not that good. It is not even used on steam very often (for "3d" games), where 20 tags are possible. With double the tags and this tag still not surfacing for 3d content, I am quite convinced, that it is useless as a tag in most cases. It is implied in too many other tags or not of interest. Or like in this thread's point, people do not use it consistent and do not have the same opinion what it actually means.