The story in Folk and Fairy Tales of Azeroth strongly implies that the sundered part of Sylvanas' soul, the one that the Jailer seems to have restored to her, is an innocent and more or less "good" aspect of her selfdom that she's been missing since her conversion into a Banshee at Arthas' hand via Frostmourne. This is likely reflective of the sense that while most Forsaken are darkened by their current state and the traumas that created them, they have an essential humanity that still clings to them, a quality that Sylvanas has always seemed to lack.
The restoration of this portion of Sylvanas' soul makes her neither good nor innocent, but it may give her cognizance of just how far she's allowed herself to fall, and the rank hypocrisy of willingly embracing the same power that ground her kingdom and people into dust. Arguably, this could be a punishment far worse than even death, but that depends on whether the narrative opts to go that route as opposed to complete redemption.