A 'D Notice' is to do with Defence issues, related to the armed forces. It's not legally enforceable, but more of a gentleman's agreement that you phone up Rear Admiral something-or-other at the MOD and run anything by him first that could compromise national security or harm servicemen's families etc.If they really want to shut you up, they've got to get an injunction served, which is legally enforceable.
They Can & Have Used "D" Notices In A Coroners Court Which Involved A Death Related To the Elm Guest House Child Abuse Scandal , So As For Not Legally Enforceable Depends Who Issues I