The ads that say "MUST CLOSE" are old, prior to the negotiations with Bernadette Peters and Elaine Stritch. Additionally, often those ads are prepaid to contractually end on a certain date. Sometimes it's week-to-week, other times month-to-month, but can really be any designated of length. Even if the show were extended or they canceled the closing date, the ad may not be taken down right away.
That being said, the production as of the date of this posting is still officially set to close on June 20. (Until they announce Bernie/Stritchy because I still have hope that it'll happen!)
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle
