Feb 8, 2018
Letter from the Congregation of the Doctrine of the Faith to the Bishops of the world
regarding the revision of number 2267
of the Catechism of the Catholic Church
on the death penalty.
https://press.vatican.va/content/salastampa/en/bollettino/pubblico/2018/08/02/180802b.html

The Roman Pontiff has authority to teach the faithful and the nations even in prudential moral matters. He does not teach that the death penalty was historically never licit, he teaches that there are no conditions under which it is now admissible.

By analogy, like a doctor who says bloodletting was once considered a reasonable practice to treat raging infection, but that prudential judgment, better understanding and treatments now make bloodletting inadmissible.

The Roman Pontiff is the successor of blessed Peter, the prince of the apostles, true vicar of Christ, head of the whole Church and father and teacher of all Christian people. To him full power has been given.. to tend, rule and govern the universal Church.
-Pastor aeternus, ch 3

If one does not accept the authority of the Supreme Pontiff to teach in prudential moral affairs, that is a serious matter. And a Catholic is well advised to ponder quietly what this might imply.