This is a datapack that overrides the default world generation to cap the terrain at sea-level for the Overworld, Nether, and End, so its kind of like having the "Overworld" Superflat preset applied to all dimensions, with all biomes and structures. Except it also has caves, oceans, rivers, and ponds.
"Aw I wish it was just like the 'Overworld' Superflat Preset, where it doesn't have caves..."
Check out my No Caves pack! It now has support for this pack, meaning you can have a caveless overworld and nether for a true superflat experience!
Please note when using this, just add the datapack on the world creation screen and create a world like normal. You should not to use the Superflat Preset.
If you wish it was more like the Classic Superflat with 4 blocks of terrain...
Checkout Superflat Dimensions
This pack should work with biome mods (although there may be some weirdness, depending on how the mod was made).
If you've never used terrain mods together, you should know that they generally override each other. The same case is for this mod.
Pre-1.21

Datapack that caps the world terrain at sea-level for the Overworld, Nether, and End.