I wondered the same thing, Bungie didn't forget about her though because there is an object on IO that you can scan and it prompts ghost to say something along the lines of "Remember our exo friend that warned us about the black garden? I wonder where she is now." I actually have this theory that Hawthorne is actually the Exo Stranger in our timeline/reality. I also made this connection because well they both wear ponchos and we know Hawthorne isn't a Guardian and the Stranger didn't seem to be one either. But I could be wrong on this, just kind of a tinfoil hat theory for fun. It would be an interesting concept for sure.