1 - Use Minecraft version 1.15 and create a new flat world. In the presets of it, put new preset :

minecraft:bedrock,10*minecraft:stone,5*minecraft:dirt,minecraft:grass_block;minecraft:plains;stronghold(count=1),village,biome_1,decoration,lake,lava_lake

The presets I put above mean :

• 1 bedrock
• 10 stone
• 5 dirt
• 1 grass block
• minecraft plains (biome plains)
• stronghold
• village
• biome_1
• decoration (tree & grass)
• lake
• lava_lake

The presets that do not work in 1.16 and higher are :

• stronghold
• village
• biome_1
• decoration (arbre & herbe)
• lake
• lava_lake

By creating the world, we can see that the presets we were validated, there are villages, decoration (trees and grasses) as well as lakes and lava lakes compared to a flat world with the basic presets where there are no there is nothing except the villages.

2 - Go to Minecraft version 1.16 or higher (1.16 / 1.17 / 1.18 pre-releases 6) and create a flat world with the basic presets and another with the presets I gave above, the difference is that in the world with the modified presets, it does not take into account decorations (trees and grasses) as well as lakes and lava lakes.

3 - Then to finish, if we convert a 1.15 flat world into a higher version of minecraft with preset changes, these are not counted from the ungenerated chunks of the map. Those who will emerge a very basic flat world from chunks not generated in version 1.16 and above.