Ive seen alot of games using this term, Public Enemy, and in most cases I also see it used to enforce the rules of most diplomacy games; though there are of course a few exceptions.

This being said I find that using PE rules kind of ruins the fun of a diplo game, because you always have this safety net where everyone has to help you or, in the case of the aggressor, you lose any strategic advantage you may have had otherwise. Plus it makes betrayal in Rp-based games seem clunky.