I remember in the olden days when Arcanis was a 3.0/3.5 system. In their books, they had a "sealed A" that meant that any such content was allowed into their living campaign. It is an idea I really appreciated, especially when we played through the excellent and quite grisly Nishanpur story arc.
I still think this is an easy way to let players know something is campaign legal without having to put anything more in the book.
Not only use this in books but also on this blog. Thus letting you know of a campaign rule or addition.
Those of you who have the book on the ArchDespotate may have seen this symbol already. I placed that in there in the section about the law and crime. But also how to avoid and minimize punishment. This is a neutral evil nation after all!