Yes, I was taught that it was not doctrine. I just searched the Catechism for Limbo and came up with nothing, although this seems to be the closest:
usccb.org/catechism/text/pt2sect2.htm#art1
I have an English Catechism from 1936 that teaches it, and a copy of the 1976 edition of “The Teaching of Christ”, which teaches it. And two editions of Denzinger’s Enchiridion - one a 1957 English translation of the 30th edition, the other the 1976 edition in Latin & Greek - that teach it. All these books have the Imprimatur. That’s not by any means a complete list of authorities which teach that Limbo is part of the Catholic Faith - even though it has never been defined.
Pius VI rebuked the Jansenist Council of Pistoia of 1786 very severely, in his Bull “Auctorem fidei” of 1794, for calling Limbo a “Pelagian fable”. And some theologians have suggested it is
de fide. The author of the article “Limbes” in the
Dictionnaire de Theologie Catholique put some of his D.T.C. articles together in a book of which an English translation was published in 1929 - he states that the Fathers of Vatican I were preparing a definition of what amounts to Limbo; only the name “Limbo” was missing, as his quotation shows.
Padre Riccardo Lombardi, the author of a book on “The Salvation of the Unbeliever” (ET 1936), assumes the existence of Limbo throughout. As do many authors whom he quoted - he discusses the who shall go there in detail, as had the writers he quotes.
As to the value of theological opinions - they are not something to despise; the teaching authority of theologians, though it is not, & cannot be, the teaching authority of the bishops, is genuine for all that. “Oh, but X is
only a theologian - what he says, doesn’t matter”, would have been incomprehensible a hundred years ago: so their judgements on the subject of (say) Limbo, though obviously not solemn & weighty enough to *define *anything (they have never had *that *weight, of course), have their own kind of authority & weight. Theological opinion is the raw matter of Catholic teaching, or used to be, so the opinion of theologians carries great weight - or used to; its value is that it testifies to the Catholic Faith, & is the opinion of those who have made sacred doctrine the study of their lives; something the bishops have not been able to do for themselves as once they would since the 12th century - they’ve had too much else to do as well. So the theological specialists entered the the scene.
I can’t be bothered to copy out all the quotations on the subject - I’m too lazy

But they exist. And there was a 1951 Allocution of Pius XII, and so on. Limbo has been taken for granted, defended, or discussed by Popes, bishops, & theologians for over 400 years - & that is a deliberate underestimate. By the standards set down in the
1998 Note accompanying
Ad Tuendam Fidem, Limbo is part of the of the Faith. So don’t be taken in by those who mistake (or confuse) absence of explicit dogmatic definition of it, for absence of Catholic teaching of it. There is plenty. (Whether this will be reflected in the decision on the topic, is another matter entirely) ##