No one does anything about it and the law becomes a dead letter.

没人执行，该法律即成一纸空文。

《柯林斯英汉双解大词典》

Rules and regulations, once set up, should not be turned into a dead letter.

规章制度一经订立，不应成为一纸虚文。

《新英汉大辞典》

Many people say that detente is now a dead letter.

很多人说缓和国际紧张局势的论调已是明日黄花。

youdao

His Administration should repeatedly declare, without equivocation, that we consider this document a dead letter.

FORBES: Fact and Comment

But the assumption must be that all sanctions under the stability pact are now a dead letter.

ECONOMIST: Europe's fiscal follies

Short of premium support a dead letter for the Obama Presidency Republicans could do more to develop a proposal called comprehensive cost-sharing.

WSJ: Review & Outlook: A Ryan Reboot