Smooth Bedrock for fabric makes bedrock generate flat in worlds. Worlds that have flat layers are configurable.
As of v1.1.3, there is a config file that can be edited. Defaults:
{
// Set to false to make the dimension whitelist act as a blacklist.
"isWhitelist": true,
/* A list of dimension ids that this mod should filter based on the 'Act as whitelist' setting.
Modded dimensionFilter will need to be added here to be affected.
*/
"dimensionFilter": [
"minecraft:overworld",
"minecraft:the_nether"
]
}