Last Sunday there was a Sondheim gala in London and he recorded an audio for the occasion saying that he is writing a new musical. It's also true that he has being writing a new musical for the past 15 years and that he said he migh give up the project with David Ives.
I've heard, apocryphally, that Sondheim has scores and scores of... well, scores for projects, unheard by anyone but a very few.
I personally think we'll never see another completed show by him, and anything he's working on he does because he enjoys doing so, not to see it produced.
"...everyone finally shut up, and the audience could enjoy the beginning of the Anatevka Pogram in peace."
I've heard the same. It's two movies with similar themes and plots being combined into one story. Supposedly it's close to workshopping, but my source there isn't solid and you can technically workshop three songs and one scene.