It's only illegal with certain types of orders. National Security Letters used by the NSA for instance to gag companies include a clause against disclosure. Certain warrants may also include a gag provision. It operates under the principle that they can gag you but under the first amendment they cannot coerce speech. So they can prevent you from saying "the feds have installed a logger on the site and your IP and content are being recorded by the FBI", but they cannot compel you to say "the FBI has not compromised our servers as of date" as that is forced speech.

Of course one issue with this is it doesn't preclude other solutions such as straight up seizing the hardware and installing taps on it and/or preventing the owner from access to it (which is why you may want to host your canary on an outside service) or more covert measures such as the NSA just busting out some malware and owning the server through a vulnerability without anyone's knowledge.

In short don't post anything that violates the laws of the United States, assume it is entirely possible at any time that this whole place could be seized by the feds without knowledge and every IP address logged and correlated with account details.

Also remember you don't just have to worry about the feds. If fascists or just plain angry liberal anti-communists (what's the difference right?) could seize this site or compromise it to expose identities I'm positive they'd be happy to so we had all better hope those running it are on their A-game and keep all server-side installed software packages up to date and patched for vulnerabilities.

France, I think