I recently posted a question regarding salt hydrolysis, and was surprised that a 'salt' tag was not available, despite it being a prominent topic in chemistry.
Is there a reason as to why this isn't a tag?
I think it is not in use, because it is a pretty fuzzy subject. Most of the time it is covered by acid/base chemistry, (aqueous) solution, precipitation, etc..
We have noticed in the past, especially in organic chemistry, that tagging with the reactants involved is inferior to tagging the questions with the reactions involved. From that point of view, salt is not a good choice as a tag.