https://en.wikipedia.org/wiki/Warrant_canary

A warrant canary is a method by which a communications service provider aims to inform its users that the provider has been served with a government subpoena despite legal prohibitions on revealing the existence of the subpoena. The warrant canary typically informs users that there has not been a court-issued subpoena as of a particular date. If the canary is not updated for the period specified by the host or if the warning is removed, users might assume the host has been served with such a subpoena. The intention is for a provider passively to warn users of the existence of a subpoena, albeit violating the spirit of a court order not to do so, while not violating the letter of the order.