A warrant canary is utterly useless as a defense.
Any secret legal order to alter IT systems (the specific threat model it is most often suggested for) can logically also include an order to maintain a fake warrant canary.
Remember the constitution only really, effectively, says whatever the current Supreme Court says it means. And really I don't think anyone want's the 100% literal 'shall make no law' interpretation of free speech; that would throw out any kind of labeling laws for starters, companies would have no government compulsion to accurately label drugs or food products for example.
On a practical basis i cannot evaluate the jurisprudence involved and I would assume the number of people who credibly can is very small, especially in the context of "secret courts for national security reasons".
A useful test would be if any of those few had demonstrated a personal risk using this as a defense and succeeded. The rest of us can only guess the risk based on the reputation of the entites involved.