Good point. How would you differentiate between 2d images and 3d renders that are static and hence also 2d images?
Would you tag such a render 2d or 3d?
Would you expect a visual novel that tags 3d to have a walking simulator environment, like a first person or third person engine?
And for playing in 3d, first person and third person are more significant. If you only tag 3d, I will not expect first person view and walking in 3d environment. Most of the time the 3d tag is superflous anyways. As useless as tagging indie here.
But let's have a look at some restricted consensus environment with tagging and the tags of Grim Fandango on Steam. It does not tag 3d, but it does tag third-person. Skyrim tags first-person and third-person, no 3d. There are games tagged with that, but I guess it often is replaced with more relevant tags, as those tags seem to be majority vote.
Since there are only 10 tags to tag, 3d is useless most of the time, imho. If you do not make a 3d puzzle, other tags have more information. Visual novels are usually in first person, but they do not tag this either. But there, 3d actually has some information, because in combination, it indicates, that the art is done with 3d models - but even here, the screenshots and thumbnail of the game will give that information even better.