What happened to Sarah Bolger's Jade on Into the Badlands? The character disappeared after season 2 of the AMC series, but her absence can be explained by the direction of the story and the state of the cast in season 3.
Jade was part of a rather complicated family in season 1 of Into the Badlands. She was the new, young fiancé of the main villain of the season, Baron Quinn (Martin Csokas). She had to share Quinn with his first wife, Lydia (Orla Brady), who was aware of Jade's manipulative nature. During this time, Jade was engaged in a secret romance with Quinn's son, Ryder (Oliver Stark). The show changed after season 1, when Quinn was believed to have been killed by the show's protagonist, Sunny (Daniel Wu). Like many of the characters, Jade's life took a big turn in season 2. Jade married Ryder and took over Quinn's barony. Jade and Ryder ruled Quinn's land together.
Jade's final episode of the series came in season 2, episode 7, titled "Black Heart, White Mountain". Quinn returned to take back what he felt to be rightfully his, and made a temporary alliance with the Widow (Emily Beecham). Together, they led their forces into the Armadillo Territory. After storming her base, they managed to easily defeat Jade. Though it was initially decided that she would be put to death, Quinn declared that she would be taken to the border and banished forever. Some fans expected Jade to return with a vengeance, but Jade's exile was actually the end of her story on Into the Badlands, with no reason for her to come back.
Sarah Bolger is currently the female lead on the FX series and Sons of Anarchy spinoff, Mayans M.C, which began airing in the fall of 2018, only a few months after Into the Badlands season 3 premiered. Her new role could easily explain her departure from the series, but it's also likely that Bolger was never expected to return as Jade in the first place.
The cast of Into the Badlands looked drastically different in season 3. Several new characters were added, and several main characters from the first two seasons were no longer around. Quinn, Ryder, and Veil (Madeleine Mantock) were all dead. With all of them gone, all the plotlines related to Quinn's family had been wrapped up, so there wasn't much of a purpose for Jade to return. Furthermore, season 3 put considerable focus on Azra and the Gift, two things that weren't connected to Jade in any way. Also, most of the characters that she had interacted with -- except for Lydia -- were gone, so there wasn't a clear place for Jade to fit into the story.