Warrant canaries are largely believed to be unworkable. Ie federal lawyers are going to say "cute, but no, you cannot disclose that we warranted you in this or any other way."
What you should be asking is their precedent for compelled false speech, which is a much more interesting and difficult to answer.
It does not feel right to call an IRS tax return "speech".
'Expression' would arguably be a better word for it, but the term of art is what it is.
https://www.mtsu.edu/first-amendment/encyclopedia/case/30/co...
They can say "don't do anything". They can't say "don't avoid doing something." That's the point if the age of the warrant canary notification--they stopped updating it. This is in effect a dead canary, they're saying they are subject to an order they can't disclose.