You have a misconception here.
Tags on itch are not official, nor enforced. While you might get reported for using blatantly false and misleading tags, most tags have such a wide range of applicablilty, that they are usually not false, even if you think so.
You can even release a game without tagging it.
So what you propose is already in effect. People could use the proposed tags and could use them according to your definition. The tag list is not a fixed list, and you can freely invent new tags. They might not get featured or suggested, but they are browseable. You need to write in the tag box and click filter.
Since you have a flat (or sligthly curved) display, any game is 2d, no matter what you think it is. So what 3d means is ambigous. It is projected to a 2d plane in the end. A renpy game featuring "3d" movie sequences or even still images would qualify. Speaking of movies. In that context, only stereoscopic qualifies as "3d" now (there is "3d" animation though) and in regard to games, there are "3d" games, that can be played in stereoscopic mode. To go even further, all games are in a virtual setting, that you could call virtual reality - but luckily no one caught on to that and vr is usually tagged for games supporting vr goggles. But you could tag a text game about a Matrix setting as vr...
In short, 3d does not explicitly mean, that the game play is calculated or shown in 3d.
Look at Steam tags for some "3d" games, if you can find the 3d tag there. They have 20 tags and the tags are community chosen. 3d is just not used that much.
What you seek you might find under 3d, or any of the the other tags people use for the concept, like for first person, third person, walking simulator, fps, fps-platformer.
The problem is, some tag first person, some 3d, some both or some other tag or they tag the dimensions of the game not at all and focus on story tags. And there is no enforcement of the usage.