Sodium, Optifine, Iris, Distant Horizons etc... will likely require a lot of work. But the people maintaining those are genuinely geniuses.

For regular mods a lot is handled by FRAPI or Forge.

The game is so insane popular the modders will adapt or new ones with appear even if it's a huge pain in the ass. Since they still support installing old versions of the game it's not so dire.

Might make some mods that support multiple versions stop at 1.20.1 but any new ones will be fine on the new ones, various API, helper mods, and surely some OpenGL->Vulkan translation ones will come in as needed.

I guess that mods like create (and all the addons made by many people) would need some effort. create has it own rendering system called flywheel that it needs for all the animated contraptions.

If they have to put in all this effort to port their mods to Microsoft's new code, what about also porting them to an open source alternative

Ah right, they hard forked Vulkan and then called it something else, just for the fuck you.

Yup, i use Prism launcher or Atlauncher and play a number of modpacks mostly older versions. It's no biggie.

With compatibility layers being what they are these days I don't think it's going to be too much of a problem.