One of the unwritten assumptions of a lot of early gaming campaigns was that they were post-apocalyptic in some fashion. Greyhawk had the Baklunish-Suloise Wars, which led to the Invoked Destruction and the Rain of Colorless Fire. The Known World had the Blackmoor explosion. Spelljammer had the Unhuman Wars. Dark Sun...well, that entire setting was a world stumbling from one apocalypse to another. Even third party works, like the Wilderlands, had a high civilization that fell apart and decayed to become warring factions.
There are time periods on Earth that would make for great apocalyptic settings. The most obvious is probably the fall of the Roman Empire, but there are plenty of others:
China, second and third century CE: the Three Kingdoms era. A peasant rebellion in 184 leads to the rise of warlords, the murder of the emperor in 185, and wars between successor states that last until 280. While there are great warriors in Lu Bu and Guan Yu, there are also crafty generals known for their strategems rather than their physical prowess, such as Sima Yi and Zhuge Liang. Estimates are that the population of China declined by roughly one-third during this time period, primarily from famine caused by the disruption of farming and trade networks.
Japan, fifteenth to seventeeth century CE: the Sengoku Jidai. In 1464, Shogun Ashikaga Yoshimasa, who has no heir, persuades his brother to leave monastery to become his heir. In 1465, Yoshimasa's wife gives birth to a son. Two officials who dislike each other end up supporting the two heirs, leading to a 10-year civil war that initiates a period of over a hundred years of sporadic fighting, assassination, and betrayal. It wasn't until 1615 that unification was truly achieved.
North America, sixteenth century CE: the colonization of America. Up to 90% of the native population died, primarily from disease. Their shaping of the land aided European settlers, since many food trees had been left standing when others were cut down for use as wood, along with promotion of useful plants that survived for the gap of years between the deaths of the natives and the arrival of the Europeans. While relatively little is known about pre-Colombian civilizations, the fantastic ideas held by the new arrivals could be the basis of a fantasy campaign.
England, fifteenth and sixteenth century CE: A disease-ravaged land, struck by plague ten times between 1348 and 1485. From 1300 to 1450, the population declined by two-thirds, with all of Wales having a population of perhaps 300,000 (its current population is 3 million). The Hundred Years' War is an on-again, off-again attempt by the Plantagenets of England to overthrow the Valois of France. Rebellions occur in 1381 and 1403. Starting in the 1450s, the Wars of the Roses pit various noble families against each other, with a feeble king (possibly afflicted by schizophrenia) unable to respond effectively to challenges. The last Plantagenet king dies in 1485, replaced by Henry VII. The Lancastrian Henry marries the Yorkist Elizabeth, and the Tudors become the ruling family for a brief century, during which there's a peasant rebellion in 1487, war on the Continent and against Scotland in 1513, rising religious tension as a tyrant king tries to have his every whim, and conspiracy (or the fear of conspiracy) everywhere.
Where else could adventurers be placed in a setting where things are falling apart (or have fallen apart), and they could explore areas that were tamed but are returning to the wild?