Especially after Vatican I, I prefer to learn from:
Pope Paul IV
"The sedevacantists base their position on the apostolic constitution cuм ex Apostolatus of Pope Paul IV (1555-1559). But some good studies have shown that this constitution lost its legal force (even sedevacantist priests recognize it: “We cannot use the bull of Paul IV to prove that the Holy See is currently vacant, but only to prove the possibility that it can happen…” (Fr. F. Ricossa, Solalitium 36, May-June 1994, p. 57-58, note 1). That which remains valid in this constitution is its dogmatic aspect. And, consequently, it cannot be made to say more than the theological argument already examined.
Yet the Code in the Gasparri edition refers in a note to the cuм ex apostolatus constitution.[Editor: The “Gasparri edition” refers to a special edition of Canon Law compiled and annotated with footnotes by the Italian cardinal, Pietro Gasparri. In those notes to the 1917 Code, he provides many links to the
sources of that very code.]
Counter-Argument 1: These notes of the code in the Gasparri edition mention the sources of the Code. But this does not mean that all of its sources are still in force!
Counter-Argument 2: The 1917 Code says in Canon 6 (5°) that the punishments that are not mentioned in the code are abrogated. Now, the cuм ex apostolatus constitution was a penal law, because it inflicted the revocation of an ecclesiastical office, and the punishments that it prescribed were not picked up again in the code.
Counter-Argument 3: There is more: even before the new Code, St. Pius X had already abrogated Paul IV’s constitution by his consitition Vacante sede apostolica of December 25, 1904 (§ 29), which declares null any censure able to remove the active or passive voice from the cardinals of the conclave. And Canon 160 of the Code declares that the election of the pope is regulated only by this constitution of St. Pius X.
Counter-Argument 4: The constitution of Pius XII of December 8, 1945, Vacantis Apostolicæ Sedis, which replaced that of St. Pius X, takes the same position on this subject: “No cardinal may be excluded in any way from the active and passive election of the sovereign pontiff, under no pretext nor for cause of excommunication, suspension, interdiction or other ecclesiastical impediment. We lift the effect of these censures for this type of election only, keeping them in force for everything else” (n. 34)."
https://dominicansavrille.us/little-catechism-on-sedevacantism-part-i/