So the law says NOT to notify users about secret subpoenas. If you do, no matter how cute you think your method is, IMO, you broke the law. Oh, but just I took a file offline. Nope.
The law can be unjust but that's another thing.
The law can be unjust but that's another thing.
The idea is that you have a message you update regularly to specify whether you have received an NSL -- you never delete the latest version. If you get an NSL you comply by doing nothing (and by your inactivity you've signalled that the warrant canary was tripped). There is a valid freedom-of-speech question (at least in the US) about whether you can be compelled to continue updating the message -- you can be forced to be silent but can you be forced to proactively lie when the NSL forced you to be silent?
Though, of course, they could subpoena the signing key for the canary and destroying the key would be destruction of evidence. A quorum system for signing might be more robust against this, but I have my doubts.
And of course quite a few folks think that warrant canaries wouldn't work in any case[1].
[1]: https://www.schneier.com/blog/archives/2015/03/australia_out...
While I do think secret warrants are unjust most of the time (and nobody can verify that they are justly applied when applied), the intent of the law is obviously to not let anyone know about the warrant, if you do you have broken the law regardless of how you did it. Any judge that rules otherwise is engaging in judicial activism.
In Australia we have an explicit law which makes it illegal to talk about the existence or non-existence of a journalist surveillance warrant (though in Australia you might not even be aware of such a warrant's existence). This means that any discussion of such warrants is technically illegal -- making warrant canaries impossible to implement here. The minimum sentence is 2 years, and it's specifically targeted toward journalists (and affects anyone who shares already-public information -- so retweeting such a story on Twitter would be a serious crime).
Warrant canaries are an effective PR move irrespective of the legality of tripping them. They only stop working as a PR move if you don't trip them after getting a secret warrant and that warrant later becomes non-secret.
The same is true about due process in the US, but there are limitations on what it can apply to regarding speech. The US government can legally stop you from speaking on certain matters via court order. But speech is explicitly separated from lack of speech in the US and are treated as two totally separate things. There is legal standing that non-speech cannot be considered as speech - this goes hand in hand with the 5th amendment and how a person's refusal to speak and provide testimony against themselves cannot in of itself be considered evidence against them. So a lack of speech cannot be considered evidence of guilt.
Also, there is a substantial body of law protecting the people (even government employees) from being forced to say anything by government. So, for others here wondering if the US can require them to keep updating it - they almost certainly cannot. There may be a way around that because the US government does have a fairly broad ability to regulate businesses so they could, in theory, pass legislation requiring businesses update this...maybe. But I doubt such a law would pass challenges as it would be challenged on first amendment grounds and the idea that other compulsive speech requirements on businesses have generally been geared toward information sharing and notifications of legal rights and other things that protect consumers. This is something entirely different and doesn't fall in those categories. Those requirements are all structured around spreading truthful information to keep consumers informed where here the government would be requiring businesses to lie - something that could easily be argued is against consumer interest.
I agree there are several theoretical reasons why warrant canaries might actually be a useful tool, but it's just as likely that intentionally constructing a scenario where you are implicitly telling people about a gag order through a bunch of hurdles would not be considered following the spirit of the law.
For instance, if you get an NSL you can't tell your family about it. When going to see your lawyer, you need to omit the reason why you're seeing a lawyer -- which is basically de-facto requiring you to actively lie to your family (because "I can't tell you why I'm seeing my lawyer" is arguably code for "I have received an NSL" if your family is aware that you might get an NSL one day).
I personally think this is massively unjust (and in Australia, we have explicit laws to disallow speaking about the existence or non-existence of any such secret warrants -- which makes even attempting to set up a warrant canary a crime with a minimum 2 year sentence).
It's an argument, I'm no fan of secret orders or the government telling you to lie or keep quiet.
The court will, either way, not be impressed by someone communicating that they got a warrant by not communicating in a previously arranged manner (this is basically communicating in a code language).
[1]: https://www.schneier.com/blog/archives/2015/03/australia_out...