In the play The Tempest, do you think Prospero is really Shakespeare saying farwell to the stage?
I think that it is really difficult to try to figure out what an author was thinking when he wrote a particular character or line. All I can say is that what you are asking could be true, but I have no way of saying that it is or is not.
You can clearly say that Prospero resembles a playwright. He makes a whole story happen and come together out of thin air, just as a playwright does. This can seem like magic when it is done properly.
When Prospero "retires" it is his "book" that he will sink -- this could be an allusion to the "book" of a play. In addition, Prospero talks about the dissolution of "the great Globe itself" which could be a reference to the Globe theater.
But Prospero's character makes sense if he is just a character in a made up story. So it is impossible to tell if Shakespeare really meant him to be a metaphor for Shakespeare himself.