With Nix users can state in their configuration what “features” their system supports, like the tags you suggest. Packages can then define what “features” they require. This helps with the additional restrictions you at times may have, on top of the hard constraints from the CPU type.
Note we have not standardized any GPU features yet because the Nixpkgs CI does not test on GPU’s. However, given the amount of scientific computing users we have, and the push to build also for things like MKL, I think it will happen soon.
I think the idea of tags or user-defined markers is a good idea, but there needs to be a forum/community for standardizing markers or it will become a mess.