How are you defining free will?
I’m defining free will as, eh, you know, that kind of thing that people are gesturing at when they talk about free will. (Or, equivalently, no, I’m not defining it.) Or, extensional definition: it’s the capacity-for-choice-flavored thing that humans have, dogs and AIs kind of have, flies maybe have tiny glimmers of, and plants and rocks don’t.
Are there any possible facts that could render free will illusory, eg. determinism?
I assert that determinism renders free will exactly as illusory as it renders temperature!
Is there any possible configuration of physics where free will us more than a stance or metaphor?
I… don’t know how I would tell whether a given universe had that property. Same if you replace “free will” with “temperature”.
Oh, or were you talking about quantum non-determinism? If so: I bet there is a deterministic physics very much like “ours minus quantum” ~= “the way people thought the world worked in 1890″, which could support human civilization as it existed when people were talking about free will a thousand years ago. In which case, people would be talking about free will in the way they do in our universe, with the same intuitions about it, and they wouldn’t have QM to fall back on to break free of determinism.
In which case, I don’t think QM can be relevant to any sort of discussion of free will that people were having before the 20th century.
(I also subscribe to an interpretation of quantum mechanics that contains no non-determinism, but I expect that to be a harder sell than the “people would talk the same way about free will even with deterministic physics” argument.)