Well, I'm not any kind of legal expert and don't have any specific domain knowledge, but it just seems logical to me that it's perfectly legal to post a warrant canary before any warrants have been served.

I was addressing the parent's comment about how warrant canaries should be illegal due to them being an attempt to get around the gag order, but doesn't that involve a gag order making a previously legal action into an illegal action and assumes knowledge that a gag order will be issued in the future?