BoingBoing has the story.
I have never quite trusted the idea of a warrant canary. But here it seems to have worked. (Presumably, if SpiderOak wanted to replace the warrant canary with a transparency report, they would have written something explaining their decision. To have it simply disappear is what we would expect if SpiderOak were being forced to comply with a US government request for personal data.)
EDITED TO ADD (8/9): SpiderOak has posted an explanation claiming that the warrant canary did not die -- it just changed.
That's obviously false, because it did die. And a change is the functional equivalent -- that's how they work. So either they have received a National Security Letter and now have to pretend they did not, or they completely misunderstood what a warrant canary is and how it works. No one knows.
I have never fully trusted warrant canaries -- this EFF post explains why -- and this is an illustration.