Netherrack, as well as other blocks that normally generate in the Nether Wastes (glowstone, soul sand, nether gold ore, etc) do not generate above y128 and below y0, with the world generation settings as: World Type: Caves, and biome selected as Nether Wastes.

Above y128, it's mostly just stone, with a very clear divide between the netherrack and stone (reference screenshot 1). Same thing happens at y0, with everything below it being mostly stone and water (screenshot 3, I have my head clipped at y0 in the netherrack, everything below is as described).