Noctrine proposed this in a 2010 comment—some approval, no action.
I think the combined tag adds no extra value. Any reasons to keep it?
I think we should get rid of 2d-physics.
I don't like the idea of "combined tags" like this, in general, as it just sets precedents for combining two concepts that are really orthogonal as far as the purpose of tags goes. Should we then have opengl-physics for doing physics using GPGPU techniques via OpenGL? Should we have d3d-xinput for a game that uses XInput and D3D at the same time? These would be absurd, and so is 2d-physics.
There are good arguments to be had that 2D physics can afford some fundamental simplifications and thus has a lot of conceptual difference from 3D physics. But the point of tags is not to sum up the whole of a topic in 32 characters or less, it's to provide a classification mechanism useful for searching and filtering questions. "Combined tags" actively detract from that use-case by requiring more tag inputs to filtering and searching UI. Whether or not the nuances of the differences between 2D and 3D physics math and simulation should be apparent from the question text itself, not the tags.
Besides, we don't have 3d-physics.
(Note that by "combined tags" I don't mean "tags with a hyphen," I mean tags composed of two or more sub-tags which are themselves valid for the site. 2d and physics are both valid sub-tags, but rope-physics is acceptable because rope isn't a thing. I'm not entirely convinced dimensionality is a useful tag on it's own, but that's a different topic.)