I actually don't know if the government could require you to falsely affirm the nonexistence of a legal investigation as part of a gag order - in other words, if you simply stopped publishing your canary message, whether you could be required to do so. Perhaps the failure to comply with such a request would qualify as an obstruction of justice if therre were a colorable risk of it impeding a lawful investigation by tipping off the subject of the investigation; law enforcement officers with an appropriate warrant or authority could require an explanation of how the canary mechanism worked, in the same way that they could require you hand over keys to one's safe, say. There's no right to silence for non-defendants such as material witnesses.