What on earth is a warrant canary?
A warrant canary is a regularly published statement that a service provider has not had a notice issued by a court as of a given date. If the notice is not updated or is removed users can assume a service provider has been served such a notice and as such is compromised. The idea is that users would be informed without a breach of an order made by a court. This is made more actionable when regular transparency reports are produced by the service provider.
In the UK, where this site is hosted, the powers concerned are regulated by the Regulation of Investigatory Powers Act 2000 as amended to date and penalties could range from a fine to up to five years in prison. In the US they are regulated by the Patriot Act 2001. Both of these regulations have criminal penalties for disclosing the existence of such notices where such disclosure is prohibited.
Tipping-off - Regulation of Investigatory Powers Act 2000, Section 54
(4)A person who makes a disclosure to any other person of anything that he is required by a section 49 notice to keep secret shall be guilty of an offence and liable—
(a)on conviction on indictment, to imprisonment for a term not exceeding five years or to a fine, or to both;
(b)on summary conviction, to imprisonment for a term not exceeding six months or to a fine not exceeding the statutory maximum, or to both.
In 2013 Apple were widely reported to have added this statment, which could serve as a warrant canary to their transparency report
“Apple has never received an order under Section 215 of the USA Patriot Act."
This document is not available from Apples website now, and Apple no longer add such a statement to their transparency reports. Perhaps this is an acknowledgement of thinking that removing such a canary from a report is the pretty much the same as notifying users that such a notice is in place.
Apple do however provide a lot of detail about the specific number and type of reauests made of them about devices or accounts/apple IDs. This is open and transparent and welcome.