This is a resource pack specifically for Minecraft 1.20.1 that backports the Bovines and Buttercups 2.0.0 textures to the equivalent Bovines and Buttercups versions for 1.20.1.
Ultimately, it's way too much effort to port to a version that's very behind in the Minecraft codebase nowadays. It's like asking a modder to backport to 1.16.5 after they had just finished porting to 1.18.2, if that gives you any idea on how much effort it would be to port.
This is ultimately a compromise for players who wish to at least have the new textures but are stuck on the 1.20.1 version of the mod.