Are these substantially important enough that we should have unity-2d and unity-3d tags for people to specify? People seem to feel the need to specify as such in their titles, so I see there might be a case for this, but I don't know well enough about Unity to be able to be sure myself.
I don't think we need such distinction in unity tags.
Also, adding a distinction between unity2d and unity3d, I feel we'd get questions bloated with redundant and inappropriate tags:
- and all of the other combinations...
unity3d was probably put in place because the 'official' name of the engine was 'unity3d'.
Note that in the past, we've been marking some unity related tags as blacklisted, or as synonym, such as inspector, as they did not add much value to the site.
I brought up some discussion on the various meta tags of Unity, and in turn, we merged a few into the unity tag.
You would not be versed in using Unity for 3D, and not be versed in using Unity for 2D. Tags are used to identify areas of expertise, and as such, I feel it is appropriate to use unity under either circumstance. At the end of the day, there is very little (if any) difference between using Unity for a 2D game or a 3D game. As such, -2D and -3D are only meta tags, being used to describe the specific context.
Ultimately, we would not benefit from differentiating between unity-2d and unity-3d.