The National Legion of Decency was a Roman Catholic organisation founded in 1933 that rated films, changing its name in 1965 to the National Catholic Office for Motion Pictures. Until 1978, one of the classifications it used was "C" for "Condemned". My question is whether at any time, and if so, between which years, this classification amounted to an official ecclesiastical ban on Roman Catholics watching the "condemned" films. By an official ecclesiastical ban I mean a statement of ecclesiastic law, breach of which would have been considered a sin throughout the Catholic Church.
I am sceptical as to whether this was so. The grouping appears to have been an organisation within the Roman Catholic Church, to the policy of which church members were encouraged to "pledge", rather than an official agency that made law. Another reason I would be surprised if it were an official law-making agency is that I have found no reference to its publishing in Latin.
For comparison, consider the List of Prohibited Books (Index Librorum Prohibitorum). Publications listed on the Index, or by authors whom it referred to as having all of their works banned (such as Emile Zola and Jean-Paul Sartre) were certainly "prohibited", with it thereby becoming a sin under the Church's ecclesiastic law to read any of them. It was in 1966 that the Sacred Congregation of the Doctrine of the Faith stated that inclusion on the Index no longer had that effect. Formerly known as the Inquisition, the Congregation was not an agency that made policy statements to which church members could decide whether or not to "pledge" their support.