Oh, of course! I hadn't considered that, my bad!
I was focusing so much about the back deploy logic itself that I forgot @availability relates to the OS version (which is tied to the toolchain, I assume fro your comment?).
I still think this is worthwhile, so my +1 stays, but I guess for the scenario I described we would need more. I guess this could build on top of this proposals implementation then, though.
P.S.: This is why I love it here! I've learned so much about language design and what all goes into it, it's really an invaluable experience. 