Palpatine had actually been the actual name of the Emperor as far back as a comic book or strip around the beginning of the '80s although his name never made it on-screen until the prequels came along.
Even earlier than that; the first reference to Palpatine as the name of the Emperor was in the first printing of the Star Wars novelization which was released in late 1976.
( However, the book also has an obscure reference to "later corrupt Emperors" plural, so technically it might not have indicated Palpatine as the name of the current
Emperor. But this was corrected by the time of the ROTJ novelization. )