Closely relates to MC-240230. This issue is also described by MC-240408, but said ticket contains less information and as such may be worth forward-resolving into this ticket.

The bug

1.18 aims to make biomes less complex by removing trivial variants which only existed to vary the height of terrain generation. The savanna plateau is an example of a remaining such biome, which only existed to create taller regions of the biome, and has few other defining differences: llamas can spawn in it and villages cannot.

How to reproduce

The screenshots attached to this issue show a single biome world generated with the same seed, shown from the same position via the command
/tp @s 281 92 152 124 29
and the only differing thing being the biome the world is generated with. It can easily be seen that the only difference in world generation between the two screenshots is the absence of a village in the plateau world, as well as minor terrain and feature differences due to changes that villages make to terrain upon generation.

Expected results

Since the savanna plateau is only trivially different from normal savannas now, it should be removed, all existing cases of it converted to normal savannas, and llamas possibly given the ability to naturally spawn in normal savannas (possibly only above a certain Y level).

Actual results

Both savannas and savanna plateaus exist, despite the differences being trivial.

Further notes

Henrik Kniberg has noted on Twitter that these biomes may be merged in future, but have not been yet presumably due to time constraints: https://twitter.com/henrikkniberg/status/1453411253603426304