Isn't this just a variation on old-games? (See this previous meta discussion.) The questions currently having that tag don't really fit into a single category anyway: even if we had a wiki excerpt for the tag, we'd have to remove the tag from most of those questions regardless of what it turned out to be. (I personally think of "classic games" as "old-style arcade games from the early to mid '80s, like Pac-Man and Galaga ... but that's my own definition, and in the context of this site, it wouldn't be helpful: there are virtually no questions that would apply to the whole group, and those that would can easily be tagged with something more meaningful.)
I think you should just tag the old/classic game with its platform for simplicity.