Outside of the appearance of Palpatine’s clone in Star Wars sequels have mostly ignored the old Star Wars Expanded Universe/Legends. Shortly after Disney acquired Lucasfilm, they declared that the old EU wouldn’t be part of the new canon, instead being considered “Legends”, but there remained the possibility of taking bits and pieces from the EU to adapt in future stories.
That possibility happened with the books and animated series. Grand iral Thrawn as a prominent character, though his place in the timeline changed to before A New Hope rather than after Return of the Jedi. This is the most obvious example of where the old EU was repurposed in the new Star Wars canon up until The Rise of Skywalker.
The fact that Palpatine in The Rise of Skywalker was a clone may not have been surprising to old Star Wars fans because it was an idea that was first introduced in the Dark Empire series in the early 1990s, in which Emperor Palpatine revived himself inside a clone body. It was one of many ways in which Palpatine could return, and seeing it adapted in The Rise of Skywalker ultimately made sense, even if the film didn't dive into the explanation for his sudden reappearance.
There were some very loose influences from the EU before Palpatine, though, but nothing close to an adaptation. The closest would be Kylo Ren/Ben Solo, but even that is a stretch. Naming a child after Ben Kenobi comes from Ben Skywalker, Luke’s son in the EU. The son of Han Solo and Leia Organa falling to the dark side comes from Jacen Solo, but outside of being an inspiration, there aren't many specifics to connect Jacen Solo/Darth Caedus to Ben Solo/Kylo Ren.
A clone of Palpatine, on the other hand, comes straight out of Star Wars Legends. The Rise of Skywalker doesn’t take it nearly as far with the cloning, but it straight up adapts an old EU story for the first time in the Disney Star Wars movies. With details changed to fit how differently the movies played out than the EU, this is as faithful of a recreation of the Dark Empire story as much as could be expected.
While most of this wasn’t directly said in the movie, instead spelled out in The Rise of Skywalker novel, there are a number of clues leading to Palpatine being a clone, rather than having survived the explosion of the second Death Star in Return of the Jedi. Whether it’s a one-off thing or a sign that more old ideas will be brought to the movies going forward, fans of the EU can appreciate getting at least some acknowledgment of that history in the Disney Star Wars movies.
