Currently we have over a hundred questions tagged both mathmode and symbols, and of the newest 15 questions, 6 concern mathematical relations.
Looking over those 6 questions, I see one at least, Symbol about not orthogonality in LaTeX, where the title would be more immediately comprehensible to me if it had been tagged something like [relation-symbols]. Likewise, I recall looking for the question How to typeset $:=$ correctly? not long ago and failing to find it, only finding it when it was referred to in a question asked since then.
I propose to create this tag, unless a better name is found, and add the tag to the ten most-viewed questions that fit it. Perhaps tags for other classes of maths symbols would also be appropriate.