Savanna plateau was used by the vanilla_layered system the same way as other hill biomes.
Hill biomes were removed as terrain shape is now determined independently from biome definitions.

The only functional differences between them are:

• A very slight variation of sky color (not visually noticably, #75AAFF vs #76A8FF)
• The "temperature" value (which according to the wiki affects grass color, I can't see a noticable difference tho)
• Savanna plateau spawns llamas (which seems like an unintended oddity as no other hill biome ever did that, but in case llamas in savanna plateau are intended, the biome definition would still be needed just for that and this would be WAI. See also MC-137140)

How to reproduce

1. Download https://github.com/slicedlime/examples/raw/master/vanilla_worldgen.zip
2. Compare worldgen/biome/savanna.json with worldgen/biome/savanna_plateau.json using a comapre program such as WinMerge
3. Make sure the biome is present in dimensions/overworld.json