Does prefer-color-scheme Limit Creative Expression?

No.

Or rather: if it does it’s your fault.

Or maybe: if it does then it’s because you’ve decided conforming to W3 expectations is more important than expressing your creativity.

Or perhaps in some cases: prefer-color-scheme further exhibits your creativity by allowing you the ability to create two variations of the same site.

But ultimately: prefer-color-scheme is a tool, you don’t need to pick it up.

Let’s be honest, the people who feel as if they are pigeonholed into supporting prefer-color-scheme are probably being limited by other factors beyond this media query. Likely they don’t even have much/any artistic/creative expression in their website.

Still, it’s not a requirement and I am going to work on remembering that (except at my day job, where it is a requirement).