dead letter

noun3

1. •
   An item of mail that cannot be delivered to its intended recipient; after some time it is returned to the sender, or destroyed.
2. •
   A law or other measure that is no longer enforced.

   • Laws are a dead letter, without courts to expound and define their true meaning and operation. — 1787
3. •
   Anything that has lost its authority or influence despite still being in existence or formally in force.