Many high fantasy settings share common themes without being derived (or plagerised) from each other.
According to wikipedia Lodoss and several other manga and resulting anime were based loosely on a RPG published in 1989 called Sword World.
https://en.m.wikipedia.org/wiki/Sword_World_RPG#Setting
I believe The Iron Wind published in 1980 is the first RPG material containing details oN Kulthea a.k.a. Shadow World which was known at the time as the world of Loremaster.
https://en.m.wikipedia.org/wiki/The_Iron_Wind
Just looking at dates, Ryo Mizono's Sword World was published nine years after TKA's Iron Wind. However, neither is the first to write high fantasy in a post apocalyptic setting. I particularly enjoyed many of the early Sword of Shanarra series by Terry Brooks, the first novel published in 1977.
My BASIC edition book for Dungeons & Dragons actually suggests novels to read on page B62 in order to inspire players and gamemasters. Among authors listed are JRR Tolkien, Llyod Alexander, CS Lewis, Lewis Carroll, Fritz Leiber, Bram Stoker. I love to add a bit of comedy to my games so Terry Pratchett would be included in my list.
Fantasy comes in more flavors than wine, even if merot/high fantasy is on the label/cover you might enjoy one specific maker more than another.