the only thing i can think of who believes the "dead Letter theory " is Catholicisms use of 2 Corinthians 3:6,,,
He has made us competent as ministers of a new covenant--not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.
Catholics believe ( correct me if im wrong ) that the word is imperfect, and a dead letter based on Cor 2 , 3:6 , until it is explained by the Supreme Pontiff, and allowed by him to be read by the laity, and only the catholic church can give the word life .
of course i don't believe that , the Letter Kills & dead letters are 2 very different statements , St Paul explains what Letter letters he is referring to which is the OT Law, further more dead things cannot kill because well frankly there dead, so the Law is Live and active