превратившийся в мёртвую букву

Makarov: as dead as mutton, dead as a door-nail (о договоре и т. п.), dead as a herring (о договоре и т. п.), dead as a nit (о договоре и т. п.), dead as four o'clock (о договоре и т. п.), dead as mutton (о договоре и т. п.)