Spock altered Starfleet's records so that Burnham and her starship were listed as lost in the battle with control.
Spock and Captain Picard officially met in Vulcan and Romulus achieved Spock's dream of unification and both races shared a homeworld called Ni'Var.
The Autobiography of Mr. Spock by Una McCormack revealed that prior to his departure to stop the Romulan supernova, the aged Vulcan hero composed his autobiography, which was intended for one specific reader: Jean-Luc Picard. In the years since TNG, Spock and Picard maintained their friendship, and the Ambassador was well-aware that iral Picard attempted a rescue mission to save the Romulan people from the supernova that was rescinded by the United Federation of Planets, and that Picard quit Starfleet in protest. This was part of the reason why Spock took it upon himself to attempt to use red matter to defuse the Romulan star from going supernova. But crucially, Spock told Picard all aspects of his life, including his earliest years on Vulcan when a young orphaned girl named Michael Burnham was adopted by his Vulcan father, Sarek (James Frain).
In his autobiography, Spock told Picard all about the events seen in Star Trek: Discovery season 2, which was a retcon of Spock's early life. The Vulcan detailed his childhood estrangement with Michael, the young Spock seeing the Red Angel, how they reunited aboard the USS Discovery, and how Burnham and the USS Discovery sacrificed themselves to jump 900+ years into the future to save the 23rd century. What's fascinating about this is Spock maintained the secret of Michael Burnham for over a century. Spock never even told his closest friend, James T. Kirk (William Shatner), that he had a human sister, much less what happened to Michael and the USS Discovery.
The Autobiography of Mr. Spock not only recontextualizes much of Spock's early life, but it also adds an extra dimension to Star Trek: Picard and Star Trek: Discovery. By the time of Picard season 1, Jean-Luc had read Spock's tale and knew about Michael Burnham and the Discovery, although he presumably honored his friend's wishes and kept this information private, especially since he no longer trusted Starfleet. In Star Trek: Discovery season 3, Michael saw a holo-image of the older Spock meeting Picard in TNG, and fans now know Burham's brother trusted the Captain of the Enterprise-D with his greatest secret.
Whether or not The Autobiography of Mr. Spock is officially Star Trek canon is a bit unclear. Most Star Trek novels are not officially part of the continuity of Star Trek's TV shows and movies, but the tie-in books to the new Star Trek series on Paramount+ involve writers who work on the shows and they are closer to being canonical. In effect, unless Star Trek: Discovery or Star Trek: Picard directly contradicts The Autobiography of Mr. Spock, then it's safe to believe that the Vulcan did tell Jean-Luc Picard all about his sister, Michael Burnham, and the USS Discovery, who now live in the 32nd century.