Star Wars: The Rise of Skywalker. This was something several tie-ins had been setting up, to a degree at least, and subsequently Disney has continued trying to fill in the gaps of Palpatine's past, exploring more of his survival and becoming a clone. Palpatine had various plans in place for after his death. He'd long established observatories on various planets across the galaxy, some of which contained Sith artifacts, and had these staffed by Sentinel droids, who were programmed to carry messages from the Emperor.
After Palpatine was supposedly killed in Return of the Jedi, the Sentinel droids sent the message to Imperial commanders to begin Operation: Cinder, wherein the remains of the Empire carried out a series of devastating attacks upon a number of planets, designed to destroy both itself and its enemies. This included worlds where Palpatine had some of his observatories, such as Pillio, eventually leading to the Battle of Jakku (another planet that housed one of the observatories), although the Rebels emerged from that victorious. This all had the benefit of helping to conceal his return from the wider galaxy, though it didn't directly stop Luke from getting on his trail. Luke had already been searching for objects related to both the light and dark side, and even searched Palpatine's observatory on Pillio after the Battle of Endor. Similarly, Luke is also believed to have been at the Battle of Jakku and found his way into the observatory there (at least, according to legend).
Fast-forward several years, and Luke senses the remaining presence of the dark side, suspecting even Palpatine himself could be involved in some way - The Rise of Skywalker's Visual Dictionary mentions "whispers spoke of a resurgent enemy from the past." Because of this, he and Lando began searching for Ochi and the Sith Wayfinder, with Luke taking a keen interest in finding the fabled Sith world of Exegol. Somewhat conveniently, though, the search for Ochi went cold on Pasaana. A few years later, Luke turned against his nephew, Ben Solo - a person Palpatine had been interested in and manipulating since before his birth. It's even hinted that it was Palpatine himself Luke sensed, with the Emperor revealing himself briefly with the hope of turning Master against Padawan, which ultimately worked, and that Palpatine may have destroyed Luke's Jedi Temple. All the evidence from the Star Wars canon, then, points to Palpatine realizing that Luke suspected him, and put these events in motion in order to keep himself hidden from Luke, who, while not unstoppable, would've been the biggest threat to his plans for returning to power.
When the reason for Luke's exile was first revealed in Emperor Palpatine's return, however, makes that decision worse. If Luke had already been investigating Palpatine's observatories, felt the renewed presence of the dark side, and even been trying to find the path to Exegol, then it makes it a little harder to believe he'd so readily give up everything and believe his own failure, rather than suspecting foul play. At the very least, Luke seemingly should have considered the possibility that Palpatine was behind it and continued his investigations further, given just how close he'd come to uncovering the truth behind the Emperor's schemes.
Of course, such a move would be hard to square with subsequent events, because Luke's exile is necessary for Ben Solo to become Kylo Ren, and for Palpatine to finally emerge as he does in Star Wars: The Rise of Skywalker. But that leaves a big gap behind in Luke's own logic; even though he acted emotionally in his decision to cut himself off from the Force and live as a hermit on Ahch-To, it doesn't fit as well if they were events manipulated, at least in part, by Palpatine. And removing the plot, although Luke thought it was Kylo he turned against, inserting the Emperor into these events makes it thematically weaker as well, because it removes a layer of the familial link that underscores the tragedy further. It does, of course, show just how skilled Palpatine is in those arts, and how he can coerce even the strongest of wills, but for Luke to not look into his potential return further after this feels like a big miss from Star Wars.
