Would it make sense for this to be part of enum vkd3d_shader_spirv_extension instead? It's technically not an extension, but inasmuch as extensions specify features I think it fits.
Yes, I think that would work. I'm not sure there's a reason to prefer one approach over the other for this specific feature though. I guess extensions might be slightly easier to enable for users of the API, although this may also be slightly less discoverable as an extension.