Since version 1.18 the oak forest biome is the only forest-type biome that will generate next to plains-type biomes. Taiga, Birch, and Dark variant forest biomes will not generate directly next to Plains or Sunflower Plains biomes. Where these biomes do generate near each other, there is a buffer between them composed mostly of the standard oak tree Forest biome.

To see on a large scale, load any seed into a biome viewer such as ChunkBase and highlight only the above mentioned biomes. When removing the standard Forest biome from view there will be a noticeable gap between all Plains and Sunflower Plains and any of the Forest variants.