Stephen Amell unfortunately didn't elaborate in his chat with TVLine on whether or not Talia would appear in the flashbacks herself, but it would make a certain degree of sense. When Sara first brought up the League of Assassins way back in Season 2, Oliver had clearly already heard of them and seemed to know that they weren't just a myth. He had to learn about the League at some point, and it looks like that could be in the Season 5 blasts from the past.