Star Wars has officially made the old backstory of Darth Plagueis non-canon. The character of Darth Plagueis was first mentioned in Star Wars: Episode III - Revenge of the Sith, and it was immediately apparent he was a key figure in Star Wars lore. According to Palpatine, he was a Sith Lord who had learned the secret of immortality - but was slain by his apprentice. That apprentice, of course, was Darth Sidious aka Palpatine himself.
Plagueis remained something of a mystery, undeveloped even by the old Expanded Universe, until Del Rey announced a novel revealing his backstory. Darth Plagueis was to be written by James Luceno, and was supposed to release in 2008 before ultimately coming out in 2012. This drew a lot of attention from Star Wars fans, because there were rumors Luceno had coordinated with George Lucas himself. And then, when Disney purchased Lucasfilm, all the Expanded Universe - all the tie-in novels, games, and comics - were declared non-canon. For all that's the case, though, many believed Darth Plagueis should remain binding in the lore. They pointed to the rumors of Lucas' involvement, and their case was made stronger when Luceno returned for a new canon book named Tarkin, and explicitly referenced events from Darth Plagueis.
E.K. Johnston's new canon novel Queen's Peril has finally settled the issue once and for all. A good portion of Johnston's novel is set immediately prior to the Star Wars prequel trilogy, including the same time period Luceno covered in Darth Plagueis. But Luceno presented Palpatine as still being an apprentice during the events of Star Wars: Episode I - The Phantom Menace; Maul was technically a Sith Assassin, recruited to the cause with Plagueis' blessing, and Palpatine only killed his Master after ascending to become Chancellor. In contrast, Johnston portrays Darth Sidious as the definitive Dark Lord of the Sith through the whole story. There's no way to reconcile these two books; Darth Plagueis should therefore be considered non-canon.
Darth Plagueis' backstory may be a mystery once again, but there's another sense in which Lucasfilm has demystified the Sith Lord. Tie-ins surrounding Star Wars: The Rise of Skywalker have confirmed the secret he discovered; he had apparently learned the art of Essence Transfer, surviving death by moving his spirit from one body to another. This was how Palpatine survived his death in Return of the Jedi, practicing the skill his Master had learned all those years ago.
This has another unexpected side-effect, though. Luceno's Darth Plagueis novel didn't just tell the story of Plagueis; it also revealed Palpatine's backstory. Now all that can be dismissed as non-canon as well, meaning the greatest Lord of the Sith - the one so powerful he brought the entire Force out of balance, requiring the Chosen One to restore it - is shrouded in secrecy once more. It will be fascinating to see whether Star Wars eventually chooses to tell Palpatine's new canon story.