Worldgen overhaul mods (Terralith, Tectonic, and many others) ship a dimension/overworld.json that replaces the overworld generator for every new world, so picking "Superflat" still gives you their full terrain. This mod makes Minecraft ignore that override when you choose Superflat, restoring the vanilla flat generator. It also strips non-vanilla placed features (mod ores, trees, plants) from flat worlds.
Normal worlds are completely untouched, your worldgen mods keep working there as usual. Only affects newly created flat worlds.
