I do not know much about the Expanded Universe. I've only seen the first six movies and played some Star Wars computer games, and to me the question arises you see in the title.
Was Darth Plagueis even known before Episode III? When Palpatine tells Anakin about this Sith legend, how do we know that Plagueis was actually his master? From Palpatine's telling, Plagueis could have lived thousands of years ago. Especially because Palpatine says, "It's a Sith legend" and legends are mostly very very old... or did fans just make the connection after Episode III because somebody thought "wouldn't it be cool if Plagueis would be his master and then was killed by his own (in)Sidious apprentice..."?