This principle is not new with Vatican II, of course. It is a long-standing tradition, maybe even "big-T" Tradition, as in Sacred...one of the three legs of the doctrinal stool of the Catholic faith, along with Sacred Scripture and the Magisterium. One often hears the liturgical duties of the Mass celebrant summarized thus: "Say the black, do the red", referring to the two colors of text appearing in the Roman Missal, the book from which the celebrant reads during the Mass.
22. 1. Regulation of the sacred liturgy depends solely on the authority of the Church, that is, on the Apostolic See and, as laws may determine, on the bishop.
2. In virtue of power conceded by the law, the regulation of the liturgy within certain defined limits belongs also to various kinds of competent territorial bodies of bishops legitimately established.
3. Therefore no other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority. (Emphasis supplied.)
It mystifies me why any Mass celebrant would presume to ignore this simple advice and amend the Sacred Liturgy sua sponte. It is "the prayer of the Church", not the prayer of any individual, whether clergy or laity. Thus, it is supposed to be the same everywhere. Sure, there are different languages in use under the Novus Ordo, but where celebrated in the vernacular, the Missal translations are approved by the Holy See, or "within certain defined limits" by a legitimately established "territorial bod[y] of bishops." The faithful are entitled to hear the Mass as it is set down in the Missal. [N.B., I would bet my next retirement check that no celebrant of the Mass in the Extraordinary Form deviates from the Missal text and rubrics.]
St. John Vianney, patron saint of all priests, please pray for conversion of heart of all clergy who, for whatever reason, take it upon themselves to "improve" upon the Sacred Liturgy.
That's all for now. God bless you and thanks for reading. Please pray for me.
Laudator Jesus Christus!