That statement, undoubtedly, is dogma, and remains unchanged. Sin of any kind cannot enter heaven. The modern reliance on God’s mercy does not rely on an idea that God allows souls into His presence still stained with original sin, but on the idea that God may have means unknown to us to cleanse souls of original sin without outward baptism. We already have the notion that God may do this for adult catechumens or other seekers who die before they can receive baptism (the baptism of desire). The recently proposed alternative to limbo suggests that God may do the same for souls that pass into the afterlife while they are still incapable of committing actual sin. Of course, we cannot insist that God always does so, much less that He must, but we know of His great mercy and desire that all souls join Him in eternity.
Notice that your quoted de fide statement does not state “limbus infantum exists,” but merely confirms that there is no second chance or purification after death for those who die outside God’s friendship (an important counter to those who misunderstand purgatory as being such a second chance).
There are other statements confirming limbo:
Pope Eugene IV ratified the Council of Florence, “Letentur coeli,” Sess. 6, July 6, 1439,: “We define also that… the souls of those who depart this life in actual mortal sin, or in original sin alone, go straightaway to hell, but to undergo punishments of different kinds.”
The letter of John XXII, Nequaquam sine dolore (November 21, 1321)
“…Illorum autem animas, qui in mortali peccato vel cum solo originali decedunt, mox in infernum descendere, poenis tamen ac locis disparibus puniendas.”
English translation:
“[The Roman Church teaches …] that the souls of those who depart in mortal sin or with only original sin descend immediately to hell, nevertheless to be punished with different punishments and in disparate locations…”
It seems that limbo, in the sense of a place or existence of natural happiness for unbaptized infants, was actually proposed as a merciful alternative to the earlier notion (from St. Augustine, I believe) that dying in original sin sends you straight to hell in the full sense. Even if limbo is considered a part of hell, it’s about as “heaven-like” as hell can get (just as purgatory is sort of a “hell-like” suburb or antechamber of heaven). So theologians have always struggled with the tension between the truth that even original sin bars one from heaven and the truth of God’s infinite mercy, especially in the case of souls innocent of actual sin. Over the centuries, proposals have gone from “yup, they go to hell, so sorry” to “they enter a state of natural happiness that might technically still be part of hell” to “we entrust them to God’s mercy, and by His action they might even be cleansed and taken to heaven.” In no case is there a change in definitive teaching, but there are different popular theories proposed to resolve the tension between definitive truths in the “corner case” of unbaptized infants.
Usagi