IMO, the old-school tag doesn't really provide much value. In some cases it's used as a stand-in for a specific, older version of a game (such as odd).
It's also incredibly subjective. D&D third edition is the edition of D&D that I "grew up on." Does that qualify third edition as "old school?" Or maybe 2nd edition, which I played a couple times before I really got into role playing? Or does nothing published after 1980 count?
I think the important question here, is under what situations would a person want to search for "old-school", specifically? It seems to be mostly related to original D&D, which already has its own tag.
Edited to add:
What if I post a question about the D6 edition of the Star Wars Role Playing Game? Should that receive an old-school tag, since it's an older version of an existing property? What about the original World of Darkness? Or the D10 version of 7th Sea (Swashbuckling Adventures)?
Does the tag still have value if it applies to games beyond just Dungeons and Dragons?