It is impossible for a Pope to mandate that no change shall ever be made in the Missal. And changes did occur before Vatican II. Pope Pius XII massively reformed the Holy Week liturgies. Pope John XXIII added St. Joseph to the Canon, and removed the Confiteor immediately before Communion.
Since the absolutely unchangeability of the Missal is not a matter of faith or morals, it cannot be the subject of an infallible, unchangeable teaching or mandate.
The Confiteor is back.