dead letter 
noun
1A law or treaty which has not been repealed but is ineffectual or defunct in practice.Why should we develop the common law to make the statute a dead letter?...
- In many ways the law has become a dead letter in Israel.
- Some people are of the opinion that the 1689 Bill of Rights is a dead letter, but the law will need to be changed in order for this to be so; and this may be a long and tedious process.
1.1A thing which is unimportant or obsolete: theoretical reasoning is a dead letter to a child...
- Communism is a dead letter, and nobody wants a return to military dictatorship.
- The socialist objective is a dead letter today, but remember, it was a liability for Labor for a long time.
- Jurisprudence, learning, and religiosity were not dead letters.
2chiefly North American An unclaimed or undelivered piece of mail.Apparently they are working at the U.S. Post Office, in the dead letter department.