How does that work then? If I were American (I'm not) is it illegal for me to simply say or publish the sentence "I have received no National Security Orders"?
Hence warrant canaries: they basically consist of creating a strong expectation of a (permitted) negative statement, such that its absence will be noticed. So far it's believed that you can't be compelled to maintain a no-longer-accurate canary against your will.
edit: Wait, I see what you're referring to. [citation needed], because as far as I know warrant canaries remain legal.