A Port of Bedrodium by kirillirik, please see it's modpage for all version prior to 1.21.1

Bedrerodium

Minecraft mod (for FabricMC).

Requires Sodium.

Small optimization by removing the invisible sides of the bedrock. For the player, these sides are visible only when necessary. (The bottom layer of polygons in the normal world will be deleted, and the bottom and top layers in the nether world).