Dead letter
Defined in 2 dictionaries — Ballentine's (1916), Bouvier (1914)
Ballentine's Law Dictionary
James A. Ballentine · 1916
An unenforced law; an undelivered letter sent to the dead letter office.
Bouvier's Law Dictionary and Concise Encyclopedia
John Bouvier; revised by Francis Rawle · 1914
Acts that have become obsolete by long disuse are often so called. See Obsolete.