After I switched to version 1.17.30, my world with flat generation became empty, I turned on the Cave & Clifs experiment. I can note that the world was not with a custom generation of the following type:

*

{"biome_id": 1, "block_layers": [\{"block_name": "minecraft:bedrock","count":1}

,{"block_name":"minecraft:dirt","count":28},{"block_name":"minecraft:grass","count":1}],"encoding_version":4}*