Well, I would say this is an oversimplification.
First, of course, you cannot possibly issue such a list because there are infinite ways to sin and people thinking up new ones all the time.
At the same time, there is nothing new under the sun, and therefore we certainly can know what sins are mortal because God gave us the Decalogue and the Church to assist us in evaluating the ever changing conditions and innovations of the times against the Decalogue.
Mortal sins are those that consist of grave matter. Grave matter are those sins against the Decalogue.
I suggest reading the entire section of the Catechism on Life In Christ (Section 3) but particularly the Ten Commandments.
vatican.va/archive/ENG0015/_INDEX.HTM
Can’t really say what you might have been looking at. Perhaps an examination of conscience. Such a list is meant to help a penitent prepare for Confession by giving them a guide as they review the Ten Commandments for areas of sinfulness. Contraception should certainly be listed, but I’ve seen various examinations that had it and others that did not list it. Could depend upon the audience for whom the list was intended.
Contraception is grave matter and a sin against the Sixth Commandment.