A national security letter (18 U.S.C. § 2709), an administrative subpoena used by the FBI, has an attached gag order which restricts the recipient from ever saying anything about being served with one. The government has issued hundreds of thousands (100,000’s, exact figure unknown) of such NSLs accompanied with gag orders. The gag orders have been upheld in US court.
This type of SECRET NSLs is warned as far more sweeping than even the web “take down” notices. Google receives only a few hundred “take down” notices from US government a year, affecting a few thousand pieces of item. But the SECRET NSLs are in the 100,000’s, by estimate alone, affecting MILLIONS of items of information which cannot be discussed ANYWHERE by ANYONE, under penalty of prison terms.
The low bar of requirement for US government to issue such SECRET NSLs is incredibly low: the US government only has to “certify” that the SECRECY is required to avoid “criminality”.