What do you mean by a failed warrant canary? In most cases there is no clear failure because there’s no clear plan in place to maintain them.
For example, if a website has a statement “we have received 0 warrants”. When was that published? Yesterday? A year ago? More? Even if it has a date, say 6 months ago. What does that mean? That they only update it every year? Or maybe there were meant to update it they just forgot, maybe they aren’t allowed to update it due to a gag order.
Due to the way each website does things differently with no clear guidelines, there isn’t actually a defined failure case.