Baptism has always been thought necessary for salvation. But that was qualified by allowing for Baptism of Desire (e.g. catechumens or others who intended to be baptized upon completion of their studies but died first), and baptism of blood (e.g. catechumens or others who believed enough to die for the Faith but were not yet baptized.)
Theologians had a problem with baptism of desire for infants, since they did not have have sufficient use of reason to desire baptism.
Limbo was postulated as a place of perfect natural happiness for infants who through no fault of their own, lacked baptism and consequently died in a state of original sin.
It should be noted that Limbo (which is still acceptable as a theological theory, which is all it ever was) does not imply some dark and lonely gehenna. Imagine the happiest you have ever been in your life, surrounded by the love of family and friends, as naturally happy as you can possibly be. That would be limbo, lacking only sanctifying grace, which is direct contact with God.
The Church has chosen to simply say that we entrust unbaptized infants to the grace of God, who can certainly save them if he wishes. The sacraments bind us, not God.