When Star Wars: The Rise of Skywalker went into production, it was confirmed by director J.J. Abrams and Lucasfilm that they would be utilizing unseen footage from both The Force Awakens and The Last Jedi to cobble together one final performance from Carrie Fisher as General Leia.
Early on, Lucasfilm boss Kathleen Kennedy made it clear that they would not digitally recreate Carrie Fisher for new scenes featuring the princess.
The script for The Rise of Skywalker had not been written at the time the deleted footage from The Last Jedi or The Force Awakens were shot, so it’s unlikely whatever dialogue is in those scenes would fit within the framework of this new story.
The rumor goes onto claim that a voice double has been brought in for additional ADR, which makes sense.