I was taught limbo was only used before the the resurrection and ascension and is now closed.
I can't remember any sources now...
But I do remember reading from solid sources that there is a Catholic traditional passed down that hell has various layers (probably where Dante got his idea to write his book). The top layer of hell is a place where those go who were "just" in the sense that they never committed a mortal sin but who never received sanctifying grace. In this layer of hell there is no pain of body but only the pain of the absence of the presence of God.
In my mind I have assumed that this would be the place where not only unbaptized babies but any naturally virtuous pagan or protestant would go who hadn't become Catholic before his death.