According to reports, during a taping of his Harmontown podcast, Harmon confirmed that he had been asked to return — but he asked that the news be cut out of the final podcast. Then, apparently, he said “Ah, f*** it. I don’t care who knows!” and joked that he’d only return if mercurial cast member Chevy Chase also returned.
As far as we know, a showrunner being fired from his own creation and then later asked to return has never happened before. However, it became quite obvious during Season 4 that despite his often odd behavior, Harmon was the heart and soul of the show.
Chase, however, will not be returning.