This is what they said about Libby, and all non central charactures in general:
Alex: One mystery that you said a month ago, in a Sky One interview, you were not going to show us is basically Libby's backstory, yet two seasons ago you basically said there was one significant missing piece from her story which is how she got from Desmond to the mental institution, and you also added that to know that answer, you had to show her story through another character's flashback. So, now that you've said that her story is done on the show, can you now tell us her missing piece and which character would you have told it through?
Carlton: Uhh, you know, again, these are not questions that we are going to answer. I think the point we were trying to make with the Libby story is that everything is graded in terms of importance for us, and, as we are doing the last season of the show, it's not going to be sort of a didactic, you know, here's a list of a thousand questions that we're going to answer. That would not make for a very entertaining show. We are focusing on what we consider to be the significant questions, and mysteries, and character relationships. That's the story that we're gonna tell. I think that the reference to Libby was more illustrative of the fact that I think, we accept the fact that in the end of the day there will, probably, you could ask a spectrum of a thousand different fans Well what question did you not get answered? and there might be a thousand different answers, but we are focusing on what we consider to be the main questions of the show and the main narritive. It's i! mpossible to tie up every loose end, and we don't really consider, honestly, Libby's story is incredibly tangential to the principle action on the show. For us, the focus of the final season really has to be on the main characters and what would generally be acknowledged as the most significant mysteries.