dead letter

noun
  1. 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. 2

    A law or other measure that is no longer enforced.

  3. 3

    (by extension) Anything that has lost its authority or influence despite still being in existence or formally in force.

Source: https://en.wiktionary.org/wiki/dead%20letter