Quote Originally Posted by PairO'Dice Lost View Post
Literally all of them except Eberron.

Dark Sun? Arcane magic screwed up the world and practicing it is forbidden on pain of death. Forgotten Realms? There are more fallen empires than existing ones, and Realms-Shaking Events change the magical landscape every few human generations. Greyhawk? The main magical empires had a magical nuke exchange, kingdoms and empires are fragmented, and magic is fading.
Greyhawk actually strikes me as a setting where the idea of magical knowledge being lost makes literally no sense, given that Boccob has both the means and the motivation to reestablish any arcane knowledge that might become lost to mortals