It is a clearly infallible teaching in the sense of Pope Paul VI’s use of the Ordinary Magisterium. I think the Church doesn’t formally “declare” it infallible because the Church is hesitant to just declaring everything formally infallible. I think (IMHO) that there are too many details to really be able to stick it down as infallible and to be able to properly treat it without more confusion.
Also, the folks that want to weasle their way around contraception by saying that Pope Paul VIs
Humanae Vitae was not an “ex cathedra” statement just want to rationalize their own behaviors and not get in line in humble obedience to the Church’s wisdom on the matter. We don’t need everything officially infallibly declared for us to believe it.
This has been the constant teaching of the Church, the Faith handed down to us by the Apostles through the Popes and Council Fathers!