Respectfully, the Church has, at some points, theorized about the existance of Limbo, but never officially taught it. I think it was postulated by St. Thomas Aquinas or Augustine, I can't remember which, both of whom were brilliant theologians, but not a Pope speaking ex cathedra. It began to take on force of teaching (lowercase) in the absence of any definitive Teaching from the Church. It was never a formal doctrine of the Faith, and the faithful never had to, nor do they now, believe in it.
Here is a good article on the seriousness of the teaching of Limbo upon the faithful, and it's history:
|The traditional Catholic doctrine of Limbo is in a higher category than that of a dismissible theological hypothesis. It is part of Catholic teaching since ancient times and is enshrined in magisterial pronouncements.
Pope Pius VI’s famous Apostolic Constitution Auctorem fidei, which condemned the Errors of the Synod of Pistoia, denounced the rejection of Limbo as “false, rash, slanderous to Catholic schools.”2
It is de fide — an unchangeable article of Faith — that souls who depart this life in the state of original sin are excluded from the Beatific vision.4