This mod makes the bedrock flat on the dimensions defined in your config.
Config:
[General]
#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:dirt", "minecraft:the_nether"]
#Set to false to make the dimension whitelist act as a blacklist.
isWhitelist = true
This mod can be used in modpacks hosted on curseforge.
Searching for the Fabric mod