According to EFF:
By using NSLs, the FBI can directly order companies to turn over information about their customers and then gag the companies from telling anyone that they did so. Because the process is secret, and because even the companies can’t tell if specific NSLs violate the law, the process is ripe for abuse.
Let's say a social media site decides to prominently display a disclaiming banner "No National Security Letters have been sent requesting any information about your account today" to every user when they log in.
Would it be illegal for such a website to display such a banner? Would it be illegal to stop displaying it to users against whose personal data such a request has been made? If it is illegal, what's the relevant case law?
I am only asking about websites created and hosted in the United States and users physically located in the United States.