I wasn't involved in the initial review, but was a separate keyword considered? Much how any T makes Existentials clear in plain English without having to guess at esoteric symbols, a clear English spelling of not Copyable seems like a very straightforward and clear spelling of the intent here.
Someone reading code with struct FileDescriptor: ~Copyable {... in it without having read this will have to stop and ponder what that tilde might mean, and while they may correctly reach the conclusion that it is exclusionary, anyone reading struct FileDescriptor: not Copyable {... in code would require no prior knowledge or guesswork.