I loved Leslie Grace in In the Heights and she is the main reason I wanted to see Batgirl. I can't imagine how difficult it must be to dedicate so much time and effort to a project and then have it thrown away.
But we know that Batgirl was connected to the existing DC universe. J.K. Simmons, who appeared as Gordon in this film, was cast by Zack Snyder for Justice League. We also know that James Gunn is writing a new Superman film and that Henry Cavill is not continuing in the role. This certainly suggests that Gunn and Peter Safran have decided to start fresh with their new universe. That being the case, Leslie Grace as Batgirl would only have been a one-off appearance even if the film did come out.
Now, there is no baggage connecting Leslie Grace to the previous universe that the public Is going to see. They could very easily find her a new role in whatever the rebooted DC universe is going to become. If so, she could probably play that role for more than just one film. Although it's too bad we won't get to see Batgirl, it would be very satisfying if we did see Leslie Grace emerge with a new role as one of the faces of the new DC universe that Gunn and Safran are building. I don't think they are obligated to go back to her. But I think she is a phenomenal actress. She could certainly audition and earn a spot there on merit, as she did last time. I have no doubt that she would be an asset to any future role she is cast in.