The Bug:
Bedrock is not being generated in the Nether, neither in the ceiling nor in the lower layers.

Steps to Reproduce:

• Go above the nether ceiling:

  /execute in minecraft:the_nether run tp @s ~ 140 ~ 0 90
  

• And below y=0:

  /execute in minecraft:the_nether run tp @s ~ -10 ~ 0 -90
  

    In both cases you will notice that there is no bedrock.