Never understood why gag orders don't just say "You can't say you received this order. Oh and by the way if we find you removed a canary, we'll just write that up as you having said you received this order".

Because the point of a canary is for it to be known beforehand. So the government surely knows about any canary too.

There must be some backwards definition of "speech" here which doesn't include all conveying of information (such as by removing previously published information), which makes it work, at least in the US (?)