Honestly, it doesn't feel like she's not meant to be relevant in the future. Setting her up as being from the Void Century holds no meaning if it isn't going to come into play at some point. Not to mention, she wasn't show to be killed in typical fashion of characters who are killed off in Flashbacks. We are just told that she died, and that has never been Oda's style when he actually kills off a character in a flashback. And the person who says she died, was Momonosuke, but how can he even know for certain when he was sent to the future? So it's not even like we are told she died from someone who actually witnessed it firsthand. The whole setup of the situation is just off.