I would assume so because

In 1998, over 120 letters that Pynchon had written to his longtime agent, Candida Donadio, were donated by the family of a private collector, Carter Burden, to the Pierpont Morgan Library in New York City. The letters ranged from 1963 to 1982, thus covering some of the author's most creative and prolific years. Although the Morgan Library originally intended to allow scholars to view the letters, at Pynchon's request the Burden family and Morgan Library agreed to seal these letters until after Pynchon's death.

So when he dies they'd release them.

Source