The character left the series at the end of Season 4 when Sophia Bush decided to depart, and she’s very unlikely to return to P.D. The show’s Upton twist could deliver that update on Lindsay without requiring Sophia Bush.
Chicago P.D. spent “Lines” continuing Upton’s descent into becoming a cop like Voight who doesn’t hesitate to cross lines, but she didn’t altogether get away with it this time, and the end result is a twist that could ultimately provide an update on Erin Lindsay.