Okay, so shortly after the Episode 9 panel wrapped up, JJ Abrams spoke with the press about the ending of the episode 9 trailer that ended with Palpatine's laugh after Luke's line from The Last Jedi was heard saying that no one is ever truly gone.
James Dyer from Empire Magazine spoke with JJ, and he took to Twitter after speaking with him, posting In case there’s any lingering ambiguity from the trailer (and McDiarmid’s appearance at the panel!), I can 100% confirm that Palpatine is back in The Rise Of Skywalker as I just asked JJ. He’s thrilled - and slightly incredulous- that McDiarmid’s presence on set didn’t leak.
Of course, John Boyega was asked about the Palpatine reveal, and he said that during filming, he asked JJ about it--bringing back Palpatine, how he meant to go about bringing back the Dark Lord. John's final comment is that JJ is a genius.
Now later, Kathleen Kennedy also spoke on the return of Palpatine, and she said that Palpatine's return was in the blueprint for a long time, but they weren't exactly sure how they would pull it off.