And to clarify, I don't think Palpatine found out his master's secret. Only that he would not have given up.Let's be clear about something: I have never suggested that the World Between Worlds is something that's going to feature in this film.
I brought it up merely in relation to whether or not Palpatine had discovered the secrets to immortality that Plagueis was purported to have uncovered, stating that if he (Palpatine) had discovered those secrets, he would have had no reason to seek out the World Between Worlds.