Do we need the relativity tag? Most questions using it are generally because it's a popular science question and the OP doesn't know which to use or because the OP used it along with general-relativity or special-relativity.
Why not get rid of it?
We have 10 questions tagged with all three. I've noticed that most of our relativity questions are about general relativity
I don't mind keeping the tag, either. FWIW it may be worth merging the SR and GR tags into relativity, but that may lead to a broad tag.
I'd be fine with removing it. I've always been somewhat in favor of getting rid of this tag (just not enough to do anything about it), because posts which both general-relativity and special-relativity apply to can just be tagged with both of those, and otherwise we just use one or the other as appropriate.
The potentially difficult thing about this will be going through all the questions tagged relativity and sorting them out into GR or SR.