Data packs can now add dimensions to a world, but naming one minecraft:overworld, minecraft:the_nether or minecraft:the_end causes it to be ignored completely with no errors.
This means data packs have no way to replace vanilla dimensions upon world creation.

How to reproduce:

1. Download the attached data pack containing settings for minecraft:overworld
2. Add and enable it when creating a new world
3. Notice you didn't spawn in a superflat crimson world
4. Optional: Teleport to a dimension with identical settings using
   /execute in minecraft:expected_world run teleport ~ ~ ~
   and see that it was created just fine