Today, we’re able to speak openly about two national security
     requests for the first time, specifically two national security
     letters (NSLs). These NSLs were accompanied by gag orders when they
     were served (one in 2015, one in 2016), preventing us from notifying
     the impacted account holders or publicly disclosing their existence.
     The FBI recently informed us that the gag orders have been lifted
     and that we may notify the account holders.
     We have provided each of the account holders with copies of the
     relevant NSLs (certain information redacted to protect privacy) as
     well as the account data we were compelled to produce. While the
     actual NSLs request a large amount of data, Twitter provides a very
     limited set of data in response to NSLs consistent with federal law
     and interpretive guidance from the U.S. Department of Justice.
     We’re encouraged by the lifting of these two gag orders and those
     recently disclosed by Cloudflare, Google, the Internet Archive, and
     Yahoo!. However, Twitter remains unsatisfied with restrictions on
     our right to speak more freely about national security requests we
     may receive. We continue to push for the legal ability to speak more
     openly on this topic in our lawsuit against the U.S. government,
     Twitter v. Lynch.

   More with links:
   [1]https://blog.twitter.com/2017/transparency-update-twitter-discloses-
   national-security-letters

References

   1. https://blog.twitter.com/2017/transparency-update-twitter-discloses-national-security-letters