Summary
- Palpatine's identity as a Sith Lord remained a secret up until the original trilogy, allowing him to manipulate the galaxy from behind the scenes.
- Darth Maul's Crimson Dawn syndicate revealed Palpatine's secret to the criminal underworld, exposing his true nature as a Sith Lord.
- Palpatine's secret lost its value after his death, leading to the bold proclamation of his return as a Sith Lord in The Rise of Skywalker.
While Emperor Palpatine's identity as a Sith Lord was originally a closely guarded secret, the Star Wars galaxy learned the truth thanks to Darth Maul. Even during the events of the original trilogy, Palpatine's identity as a Sith was largely unknown. This was particularly advantageous for Palpatine aka Darth Sidious, though it was a benefit he lost following his return in The Rise of Skywalker.
Following the Republic's transformation into the new Empire seen in Revenge of the Sith, Palpatine let the galaxy believe that he was nothing more than a kind and benevolent ruler. Instead, it was his advisors and those serving beneath him who were the true villains responsible for the Empire's oppression. This was a darkly brilliant move of strategy, ensuring that acts of rebellion would often be aimed at the wrong targets. Likewise, the grand-scale deceit more than likely amused Palpatine, especially after he dissolved the Imperial Senate in favor of ruling completely through fear with the new Death Star. However, Palpatine's true identity as a Sith eventually came to light.
Your browser does not support the video tag. Darth Maul's Crimson Dawn Revealed The Truth About Palpatine 
Having escaped the custody of Ahsoka Tano and the 333rd Company during Order 66, Darth Maul survived into the Dark Times of the Empire, establishing his Shadow Collective of criminals into an official syndicate known as Crimson Dawn. In Solo: A Star Wars Story, Qi'ra became Maul's new second-in-command following the death of Dryden Vos. Furthermore, the comic event Crimson Reign by Charles Soule and Steven Cummings confirms Maul shared his plans and schemes with Qi'ra to eventually destroy his former master and his new apprentice Darth Vader, confirming their existence as Dark Lords of the Sith.
In the aftermath of Empire Strikes Back, Lady Qi'ra took this information from Maul and shared Palpatine's secret with several criminal elements in her service, having become the new head of Crimson Dawn following Darth Maul's death (as seen in Star Wars Rebels). Although Qi'ra fails to kill the Sith Lords and achieve Maul's dream in the subsequent Hidden Empire event from Soule and Cummings, the truth about Palpatine was exposed to the criminal underworld nonetheless. This occurred in the months leading up to the events of Return of the Jedi where Palpatine did indeed meet his demise at the hands of Vader himself.
