Basins (or erosions) are as intended according to MC-1242, MC-212606, MC-229112. They were common before but were removed unintentionally in 21w41a (1.18).

Basins generated in all overworld biomes before. But, when surface rules were added in 21w41a, they were somehow limited to only two biomes: frozen ocean and deep frozen ocean.

Here is also some information about it.

This bug completely blocks MC-130788.

How to reproduce:

Create two single biome worlds with frozen ocean or deep frozen ocean and another biome. Then check if there are basins.

Example:

Seed: 1
Coord: 406 85 453
Biomes:
Frozen ocean and deep frozen ocean: A basin is generated around 406 85 453
Any other biome: There's no basin at all
Due to the fact that ocean biomes are almost never generated on land, there're almost no basins in default worlds.