"A warrant canary is a method by which a communications service provider aims to inform its users that the provider has not been served with a secret government subpoena."
Basically, it's a static page that states that a service has not been issued a secret government subpoena as of a certain date. The date is then updated regularly. If a secret subpoena is issued, the canary is not updated alerting users to possible compromise of their privacy.
Tutanota email service is a perfect example and even goes so far as to say the number of requests sent and requests granted.
It is now nearing a year since this question has been asked. Has there been any developments in this regard?
Fantastic idea IMO!
Vaultoro prides itself as being one of the most transparent exchanges in the world. A warrant canary would definitely add to this mission. This is a great idea and we will look into it.