I've been having an interesting discussion about post-apocalypse vs. disorder in the comments of this post. Basically, are fantasy games like D&D post-apoc?
Personally, I think most fantasy settings assume disorder more than post-apocalypse.
Disorder assumes that the situation is in flux. There is enough of a breakdown in central order that small bands of adventurers can find a use for themselves. Yet there is enough civilization out there to sustain a (often mostly off-camera) support network for the same. Things may have been better in the past, and knowledge may have been found and lost, but generally the tech level is about the same. Access to magic may have changed, but PC's interaction with it will be largely encounter relics meant to give them a boost in power (or an interesting challenge) without allowing anything to be replicated. Yet, overall, civilization exists even if it's not very calm, peaceful, or well-organized.
Post-apoc, to me, assumes a drop in technology level* and a severe change in the setting. A period of small technology drop can just be a big period of disorder. This is much bigger - civilization as a whole, across the entire game setting (with rare and often inaccessible exceptions), has been trashed. High tech (and possibly magic, too - think Tekumel's setting here or Thundarr) is available but the level of ability to replace it has dropped precipitously.**
Post-apoc thrives in the "rebuild civilization" mode and the "damn it all to hell" Hestonian/Maxian vibe. It's not civilization wilderness that may have been civilized in the past but isn't now, but rather civilizing everything. It's all wilderness except for those pockets. Greyhawk and its Suel/Baklunish war may have been metaphoical Cold War gone hot with magic swapping in for nukes, but the lands of Greyhawk are civilized (yet in just enough disorder to be primed for war and adventuring on the seams.) The occasional crashed spaceship notwirthstanding, it's lacking that "the whole world has gone to hell" feeling of post-apoc. Settings of this sort are generally as post-apoc as Futurama. Disaster struck in the past and reduced humanity to spears and sharkapults, but it's back to civilization. Even Tekumel is just science mixed with fantasy - for all of the post-apoc nature of the setting, it's just a background explanation for the monsters and the magic and the science-based magic items. It's not really post-apocalypse as it is disorder bolted onto a mix of science and magic mixed together.
It's this combination of tech loss* and reducing civilization to tiny pockets (or to nothing) that makes for post-apocalypse, not just if an apocalypse occurred in the past. Disorder is more localized. Note that during the Dark Ages in Europe, for example, access to knowledge fell, technology changed for the better and the worse, and major powers were broken into many smaller powers (basically). Yet civilizations outside of Europe continued to develop. Disaster wasn't worldwide and utterly destructive. Most fantasy games take this tack, and assume that even if things were better before there are still glittering cities to spend your loot in and decaying grandeur of the past and bright new futures being forged. You can still hire soldiers, swear fealty to distant lords, and buy things from far off lands.
One comment I've seen before is that the random encounter tables tend to show that even in civilized areas, you get pirates on the high seas, bandit armies roaming about, and monsters. But I'd argue the random encounter tables of fantasy games are skewed to the interesting, not the statistically accurate representation of society. They assume that there is a role for armed vagrants like PCs, especially those who'll tame the wilderness, beat the bandits, and rise to prominence at the point of a sword and through the strength of their arms and magic. They assume that's where the players are, too - even if they never suggest there are better, more civilized places, it's likely because the game doesn't take place there. After all, fantasy RPGs tend to leave a lot of blank, uncovered stuff and focus on monsters, treasure, characters, and violence because that's the game they're meant to provide rules for.
I think if you assume any game that assumes that, in the past, things were better in some ways and wars and death and disaster changed that are "post-apocalypse" you're in danger of making the term too broad to be useful. If D&D is post-apoc and so is Gamma World, then what's the difference? Magic? Gamma World swaps in sufficiently advanced technology - and a crossover game can easily mesh the two. Truly post-apoc fantasy (like, say, Prince of Thorns) exists, and lets the characters interact with the "lost" technology in a very direct way. It's not just color to explain why the writer isn't explaining how things work, it's part of the basis of conflict and action.
Short version? Post-apoc has pockets of civilization and widespread and deep destruction. Disorder assumes civilization exists and people adventure in the disordered areas or during periods of war. Even shorter? Post-apoc has nukes you can set off, disorder has ones you can't. (Mostly joking.)
* Lost magic is an interesting variation on lost technology, but generally it's "lost" to allow the GM to make unique magic items and spell effects and weirdness available without allowing PCs access. That's not really necessary - you can play a modern game where there are nuclear weapons and not let the PCs play with them, so you could really make a fantasy game with super-powerful spells that take so much precision, time, power, and skilled manpower that it's unrealistic for PCs to have access. It doesn't actually have to be lost. Being lost is just a way of saying, "I don't want you to be able to do this but I want it around."
** For this reason, I think of Car Wars as disorder, not post-apoc. Mad Max helped inspire it, but you order heat seeking missiles and lasers from Uncle Al's and compete in organized duels.