I bet there's a great organization that works on defending civil liberties in the digital world. It turns out they have an article on this, from a full decade ago. It also covers who else is using canaries, and existing, highly limited, precedent for compelled speech.

https://www.eff.org/deeplinks/2014/04/warrant-canary-faq

https://www.eff.org/deeplinks/2016/05/canary-watch-one-year-...