spoilers for TCW seasons 5-7
The last we see of Barriss Offee in the Clone Wars is her being arrested in the finale of season 5 for framing Ahsoka. After that, we never see her again, not when Ahsoka returns in season 7, and we don't hear about what happened to her in Order 66. Barriss is never mentioned by any of the other characters in any media since - not in Rebels, not in season 7. Only in the Ahsoka novel does Ahsoka mention Barriss, but it's an offhand remark about how Barriss betrayed her that is unrelated to Barriss's fate.
We know from Rebels that Luminara Unduli was killed, but as she wasn't Barriss's master at the time that doesn't really give any answers.
People have theorized that she probably was just killed by clones in her prison cell, but this hasn't been explicitly confirmed, even if it does make the most sense. Is there anything about what happened to Barriss after season 5?