RPG SE uses system-agnostic a lot, and in a rather esoteric fashion.
Its tag wiki, however, is a sad, sad thing:
Not related to any one particular system of rules.
This really needs more information. Users should be able to look at the tag wiki to find out which kinds of questions need the system-agnostic tag (vs. a specific system tag or no system tags at all), as well as obtain answers about how to structure their answers in a "system-agnostic" way. See Tag Wiki Best Practices.
(Why don't I just do it? Well, I've got some "I know it when I see it" intuition about the tag, but I really can't figure out how to express the answers to these questions in clear words that other people can understand.)