We order that the prescriptions of this Constitution go into effect November 30th of this year, the first Sunday of Advent. We wish that these Our decrees and prescriptions may be firm and effective now and in the future, notwithstanding, to the extent necessary, the apostolic constitutions and ordinances issued by Our predecessors, and other prescriptions, even those deserving particular mention and derogation. - Paul VI, “Apostolic Constitution” Missale Romanum (1969)
And those that did not or put up a fight over it were reprimanded and/or disciplined.
So effectively? Yeah, it was forbidden.
That is precisely what he did NOT make clear.
There isn’t a syllable there that even suggests that the old Mass is forbidden. All Paul did was promulgate a new Missal. All that flowery language refers only to the promulgation of the Novus Ordo.
He knew he didn’t have the authority to abrogate the old Missal, and he never did. He knew that bishops were punishing the celebration of the old Mass, but he never did more than wink at this.