Nearly every D&D setting is some stage of post-post-apocalyptic. There was a high magic civilization somewhere in the past, it blew up for Reasons but left the countryside littered with ruins full of powerful magic items and deadly traps, and rather than dealing with the immediate post-apocalyptic scrabble for survival we're far enough along that things have mostly stabilized and people are delving into those ruins for personal profit.
Different settings have the sliders set to different points, and the emphasis can be adjusted. But there's a need to explain why "ancient magical artifacts" are as good or better than anything new being produced, and why there's abandoned complexes full of treasure and monsters and traps just waiting to be delved, so a post-post-apocalypse situation is nearly always included. Sometimes multiple, either regional or cyclical.