What I did:

I took a default settings JSON from a world and changed a single character in the nether's dimension preset line. I then used this file to generate a custom world.

What I expected:

I expected the game to just ignore the nether's custom settings and continue with default generation, since that dimension section was 'broken.' I thought that all three dimensions would've generated normally(only the nether was modified on the JSON).

What Happened:

The game complained about the JSON, but still let me continue. When the world generated I discovered that both the nether and end were completely gone from the world. The overworld appeared to generate as normal with the exception that portals would never take you to their respective dimension(as they no longer existed).