It doesn’t really. Server side mods don’t touch rendering code at all, and most client side mods also don’t come anywhere near it. I last did Minecraft mod development some 7 years ago but even then you would basically never reach into the raw drawing calls unless you were implementing shaders or something.

Considering the vast majority of mods are just adding some items or creatures, they don’t need to worry. This won’t be more than the regular API changes in between versions that they’re already used to, unless it’s a more graphics heavy thing like a shader mod.

Also, even with shaders, it’s fairly straight forward to port a shader from OpenGL to Vulkan (for the most part Vulkan just gives more flexibility in that regard). The stuff around it is the hard part.

No, it only breaks the eye candy extensions.

There's a whole community that plays on private servers and uses extensions for stuff like access control, new game mechanics (which doesn't mean new shaders but new behaviors in game) etc.

The native windows version is not moddable as described above. And probably will never be because MS wants you to rent "servers" from them.

So most "serious" minecraft players ignore bedrock.