It should be clear from the above survey of relevant Catholic magisterial statements that those who now talk about Limbo as only ever having been a mere hypothesis, rather than a doctrine, are giving a very misleading impression of the state of the question. They are implying by this that the pre-Vatican II Church traditionally held, or at least implicitly admitted, that an alternate hypothesis for unbaptized infants was their attainment of eternal salvation Heaven. Nothing could be further from the truth. Limbo for unbaptized infants was indeed a theological hypothesis; but the only approved alternate hypothesis was not Heaven, but very mild hellfire as well as exclusion from the beatific vision! In short, while Limbo as distinct from very mild hellfire was a hypothetical destiny for unbaptized infants, their eternal exclusion from Heaven (with or without any pain of sense) at least after the proclamation of the Gospel, and apart from the baptism of blood of infants slaughtered out of hatred for Christ this was traditional Catholic doctrine, not a mere hypothesis. No, it was never dogmatically defined. But the only question is whether the doctrine was infallible by virtue of the universal and ordinary magisterium, or merely authentic.
http://www.remnantnewspaper.com/Archives/archive-2005-1215-limbo.htm
http://www.remnantnewspaper.com/Archives/archive-2005-1215-limbo.htm