::Not every work of a non-Catholic was forbidden, but only, among others, every work on religion by a non-Catholic.
I don't know whether the Opus Dei adheres to the index or, as it is said, censures new books, but I may say that if, it wouldn't be to blame for it. It has every right over its membership and followers, until forbidden so by the Holy See, that the Holy See itself over all the Church had when it did publish the index.
Though this has been officially denied, I believe the action of H. Em. Bertone against the Da Vinci Code was not far from censuring, and a quite just censuring too according to what one hears about its content (I actually didn't read it, Michael Crichton is better).
That Nietzsche was not included (I'm not sure about Schopenhauer) has I believe the easy reason that there was no need of clarification that his works are non-Catholic, and so "on the index" by general rule without any need of explicity. Of course theoretically, those books Kant wrote on religion (and that includes the Critic of pure r.) would be implicitly on the index as well because he was not a Catholic, but from the very fact that he did believe in God, the Congregation may have felt the status of his books, from the Catholic point of view, needed further clarification. Also, not every work of Kant was listed.
As to its importance as of now, I agree (as a layman and non-theologian) that reading some of the books today not only does not lead to ecclesiastic penalties (as should be sure) but also is not sinful in itself, as no longer any disobedience to Church law (likewise, of course, as to Card. Bertone's semi-censuring) but it may be dangerous and thus imprudent, which can be not free from sin. (?) 84.154.109.57