You're misunderstanding.

> if the delivery conduit uses AV1, you can optimize for it

You could, in theory, as I confirmed.

> It's already built, and Netflix is championing this delivery mechanism.

No it's not. AV1 encoding is already built. Not a pipeline where source files come without noise but with noise metadata.

> and provides the best possible image quality?

The difference in quality is not particularly meaningful. Advanced noise-reduction algorithms already average out pixel values across many frames to recover a noise-free version that is quite accurate (including accounting for motion), and when the motion/change is so overwhelming that this doesn't work, it's too fast for the eye to be perceiving that level of detail anyways.

> This scheme loses a major opportunity for new productions unless the director can provide a clean master and an accompanying "grain track."

Right, that's what you're proposing. But it doesn't exist. And it's probably never going to exist, for good reason.

Production houses generally provide digital masters in IMF format (which is basically JPEG2000), or sometimes ProRes. At a technical level, a grain track could be invented. But it basically flies in the face of the idea that the pixel data itself is the final "master". In the same way, color grading and vector graphics aren't provided as metadata either, even though they could be in theory.

Once you get away from the idea that the source pixels are the ultimate source of truth and put additional postprocessing into metadata, it opens up a whole can of worms where different streamers interpret the metadata differently, like some streamers might choose to never add noise and so the shows look different and no longer reflect the creator's intent.

So it's almost less of a technical question and more of a philosophical question about what represents the finished product. And the industry has long decided that the finished product is the pixels themselves, not layers and effects that still need to be composited.

> I wish Netflix could go back and remove the hideous soft-focus filtration from The West Wing, but nope; that's baked into the footage forever.

In case you're not aware, it's not a postproduction filter -- the soft focus was done with diffusion filters on the cameras themselves, as well as choice of film stock. And that was the creative intent at the time. Trying to "remove" it would be like trying to pretend it wasn't the late-90's network drama that it was.

Netflix’s AV1 Journey: From Android to TVs and Beyond