A Grand Central spokesperson confirmed the expiration, and HBG has a limited time to sell their remaining stock, until April 25. The email suggests that over time Grand Central has sold more than 20 million copies of the book, saying that "more than two-thirds of the 30 million copies sold worldwide since publication have been Hachette's low-priced edition." (The current regular mass market book lists for $8.99, though GCT added a mass upperback edition priced at $9.99 just a few months ago, in mid-December.)
A Harper spokesperson told TNR they will continue to sell their existing editions of the book -- in trade paperback ($14.99) and ebook ($10.99) -- without indicating any plans to issue a new mass-market version of their own. "Like many American classics, To Kill A Mockingbird's primary paperback format will be the trade paperback edition."
The correspondence seen by TNR indicates that the decision to not have the book available in a mass market edition is "per the wishes of the author's estate," though the write-up does not consider the idea that the estate no longer wants to see mass-market proceeds split between their principal publisher (Harper) and a licensee (Grand Central).