There’s actually quite a debate among RCCs about whether Balthasar was encroaching too far against previously established ecclesial proclamations.
It is generally agreed that the Latin synod under the patriarch Menna, which picked up and affirmed the anathemas of Emperor Justinian against Origen, was confirmed by Vigilius in signature as “Supreme Pontiff” at that time (543).
Canon 9, the final entry, bluntly states: “If anyone says or holds that the punishment of the demons and of impious men is temporary, and that it will have an end at some time, that is to say, there will be a complete restoration of the demons or of impious men, let him be anathema.” (Relatedly canon 7 denies that Jesus was crucified for the sake of demons, although the phrasing is such that specifically it is denying that Jesus will be crucified at some future time for demons as He once has been already for sinful men.)
Not long afterward, Pope Pelagius I (writing his statement of faith in the letter “humani generis” to Childebert I, April 557, explaining the Catholic faith to that monarch) asserts again as part of his opening statement (indeed as part of his “faith and hope”!) that the wicked shall be given over by God to the punishment of eternal and inextinguishable fire, “that they may burn without end.”
Pope Innocent III, writing to Andrew the Archbishop of Lyons in January of 1206, mentions in passing as part of his explanation for why the RCCs insist on infant baptism, that while the punishment of original sin is (only) the deprivation of God (thus infants who died unbaptized are not tormented with everlasting hell but only with everlasting deprivation of God), the punishment of actual sin is torment of everlasting hell. (And by ‘everlasting’ he certainly means ‘everlasting’, since his whole point is that God seeks the salvation of even the smallest child and so has made provision to save as many as possible through infant baptism.) He also, sort-of in passing, tacitly dismisses the notion that the punishment of hell may be intended to lead to the salvation of those who suffer the punishment, by appeal that it is contrary to the Christian religion to force conversion on anyone, including by torture, seeing as someone may by torture make only a shallow conversion in order to avoid the torture (but that anyway the whole operation is the worst kind of tyranny unfit for any Christian ruler, much moreso for God.)
Innocent IV, during the 13th Ecumenical (Catholic) Council, (Lyons I, 1245, convened against the monarch Frederick II), wrote in a letter (“Sub Catholicae”) to the Bishop of Tusculum, of the Legation of the Apostolic See among the Greeks (i.e. the Greek Orthodox, concerning admission of Greek congregations into Roman Catholic communion), required that they reject even the possibility of universalism and shift their notion of purgatory into agreement with RCC dogma on that topic:
“Moreover [after asserting RCC doctrine about purgatory being a transitory fire not intended to save criminal or capital sins] if anyone without repentance dies in mortal sin, without a doubt he is tortured forever by the flames of eternal hell.”
(A note to this document does state that Innocent was not sending out dogmatic decrees at this time. But he treated the matter as already being settled RC dogma.)
Clement VI, Sept of 1351, in writing the letter “Super quibusdam” to the Consolator, the Catholicon of the (probably Jacobite, i.e. Oriental Orthodox) Armenian Church at that time, who was apparently seeking ecumenical audience, while busily asserting the primacy of Roman Pontiff, wanted to know if the Consolator was willing to agree (or perhaps was already teaching) “that all who have raised themselves against the faith of the Roman Church and have died in final impenitence have been damned and have descended to the eternal punishments of hell.” (Which he afterward sharply distinguishes from purgatory.) Later in the same letter, he indicates that the Armenians are in error for believing (much less hoping) for salvation from hell per se.
Eugenius IV, issuing the papal bull “Cantata Domino” in regard to Jacobite Armenians during the Council of Florence in 1441 (re-issued in what was then ‘modern style’ 1442), ended the bull with the assertion that unless a person died in 'the bosom and unity" of the Roman Catholic Church, that person could under no circumstances be saved afterward, but that all those who die while not living within the RCC “not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart into the everlasting fire which was prepared for the devil and his angels.”
Beyond this there are several papal affirmations of the Nicene Creed portion concerning everlasting judgment, punishment or hell; by which in context with other proclamations on the topic since Vigilius (at least), they mean everlasting in an unbroken and unbreakable ongoing sequence.
I wasn’t able to find anything else in the current “Sources of Catholic Dogma” on that topic post, up through Pius XII (20th century); maybe Benedict or JP2, or one of their predecessors had something more to say on the topic.
But it’s going to be tough for Catholic universalists to hold even a hope for universalism while being in formal communion with the RCC, with this kind of dogmatic track record behind them. There really isn’t much room for debate (actually no room) as to what the papal teaching has been since the 6th century, without allowing for papal fallibility somehow.