I'm not a lawyer, but every time I've talked to one about this type of subject, they've informed me that these "clever" loopholes like warrant canaries are not

I'm not a lawyer, but every time I've talked to one about this type of subject, they've informed me that these "clever" loopholes like warrant canaries are not viewed favorably by prosecutors or judges. They'll happily slap you with conspiracy/accessory/obstruction/contempt/whatever charges if they think you're being cute and trying to short-circuit a legal order with loopholes. Apparently the law, at least in most areas, has anticipated the existence of smartasses and provided mechanisms to deal with them. I can't imagine that warrant canaries would be much different in practice. The judge would probably say that removing the canary is tantamount to revealing the existence of the letter, is thus a violation of the gag order, and that doing it will land you in contempt. Again, pure speculation, and not a lawyer.