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.