I am kinda against explicitly foreshadowing Palpatine’s return in any of the first eight movies (Palpatine’s immortality interest, Vader’s “Death Star’s insignificant next to the Force power”, the music, Snoke’s similarities to Palps, and “Show me AGAIN, the power of the darkness” don’t count as “explicit foreshadowing”).

The whole point of TLJ is, by the end of the movie, that hope has been reignited, that the First Order will be destroyed, Luke even inspires the galaxy with his return, having come to terms with his legacy. The broom boy ending shows that Luke’s inspired hope, and the boy himself is hopeful, even in slavery.

If Palpatine had been set up in TLJ, it would take that way, that hope would all be meaningless since we would know that Palpatine is just going to come back and ruin everything.

EDIT: I think I’m on the verge of changing my mind. TLJ was always going to have a sequel, and it’s literally Episode VIII. It could never have been an Episode IX. Plus, I think this criticism might be hypocritical enough to be applied to the Marvel movies and such.