Just an idea: Maybe require one or more of the following tags: 5, 6, 7, 8, version-independent? Possibly the last never with any of the numeric ones?
It would make clear, "in a single, reliable place on the page", what author thinks about version dependency of his question and give people willing to answer a clear indication if all answers are welcome, or only for some of available versions.
I have nothing against questions that does not specify version. I just don't like ambiguity, and don't like answers voted down only because somewhere deep in text question author indicated another version, than the answer author used in his solution. So it should be clear that version was omitted on purpose if that's the case.
On the other hand, if question without version tags is supposed to mean version-independent, please provide that information in FAQ or help page, to make it clear for the ones that does not dig thorough old meta (or simply fails to stumble upon such posts).