I'm getting on the old side and I have no idea what makes this tag different from the "odd" tag. Is it supposed to represent a certain style of game? if so, does it really need a tag? This seems a horribly subjective tag.
EDIT: And what is OSR?
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?
Here are my thoughts on tags.
In general it is used to when a question could applies to all of older editions of D&D. Probably including the oldest games like chivalry & sorcery, tunnels & trolls, Runequest, Traveller, etc.
But the focus in on D&D and while the different editions the same there are differences. They all need tags including one to encompass all of them at once.
Based on the criteria in the StackOverflow blog post, The Death of Meta Tags, I think the [old-school] is a meta tag. Since these are now "explicitly discouraged", I think that it should be removed.