In GL’s 1981 revised rough-draft ROTJ script, the Emperor not only knows who Yoda is, he knows him on sight when Yoda’s Force ghost appears in his underground throne room:
The Emperor turns around to see the shimmering image of Yoda standing before him.
EMPEROR
You!YODA
Evil one, this will be the last day of your existence.
A further reminder that ROTJ could have been much worse.