.....Limbo which came out of Purgatory was first mentioned by Catholics in the early Twentieth Century
No, it did not.
When Augustine wrote about the absolute necessity of Baptism, he left a note in the margin (Limbus) to the effect of, "What about infants?"
"Limbo"comes from the note in the Limbus, and has never been Catholic theology or teaching (notwithstanding the number of sisters that may have taught it to their classes).
A note of clarification. It is important to note that "Limbo," as in the "limbo of children", simply refers to Hell; however it specifically refers to the state of those who suffer the poena damni
, i.e., deprivation of the Beatific Vision, but are either exempted from the poena sensus
, i.e., the sufferings inflicted in Hell in proportion to one's personal sins, or else suffer mitigated sufferings (theologians are divided). Unbaptized children who did not attain the use of reason belong to this category.
While never the direct subject of a dogmatic definition, limbo was certainly the general teaching of the Roman Church at least as far back as St. Augustine - the only change was that the African Fathers believe unbaptized infants suffered the poena sensus
, perhaps in a mitigated form, whereas by the time of St. Thomas Aquinas, it became more common (though not universally) to teach that they suffered the poena damni
Besides the Council of Florence and the Council of Trent speaking on the necessity of Baptism, the only magisterial references I can think of are Pope John XXII's 1321 letter Nequaquam sine dolore
to the Armenians (old Denziger 493a) and Pope Pius VI's 1794 bull Auctorem Fidei
(ibid., 1526). The former does not use the term "limbo," but is pretty clear on the basic idea, while leaving the question of mitigated punishments unanswered; the latter describes the position that Limbo is a "Pelagian fable" to be "false, rash, injurious to Catholic schools."
The doctrine has certainly fallen out of favor, but it has not really been replaced by any clear teaching on the subject; the modern Catechism relies on the mercy of God while essentially ignoring many centuries of near-unanimous teaching by the Roman theologians - a unanimity that before Vatican II was often understood as a sign of infallibility.