All tags describing categories of words or sentences are pluralized expressions, conjonctions, pronoms, verbes, noms, questions, etc.
Adding the tag interjections on some question, I got stuck on this message:
Creating the new tag [interjections] is not allowed since the tag [interjection] already exists. If you think this new tag should be allowed, raise it on meta.
This was an unexpected exception. Can we turn it to a plural?