Heh. I did put that in there deliberately, but no, not that I know of. There's nothing incredibly technically difficult in making this work:
@dynamicallyAvailableBefore(macOS 10.12)
var temporaryDirectory: URL { … }
and having the compiler check that the signature matches up and everything before generating the code you wrote above. But it'd be a proposal that needs a lot of careful thought—how does this affect how people use framework code? What happens on new platforms? What if two people do this?