Warrant canaries are a legal work-around for gag-orders. If a court orders you to maintain silent about something, you can't warn your service's users about that but nobody can prevent you from publishing that you had not ordered to be silent about anything prior to the fact, so some companies put out such a document called a warrant canary. To not renew it means you would have to lie to renew it and a court can't compel you to lie. They can compel you to not remove the canary though, so ... it expiring is as much of a signal you might get if something shady is going on.