[jellicle]

Nearly nothing has entered the public domain in the U.S. in decades, and likely nearly nothing ever will again.

[VonGuard]

There was that time in the 70's when you had to re-up your Copyright, and a lot of stuff fell out, like Night of the Living Dead.

[wtallis]

Night of the Living Dead failed to ever have copyright protection due to lack of a copyright notice or registration, not from a need for renewal. Renewal became automatic in 1964, and registration or notice became optional in 1989. (And in 1976 it became possible to register for up to 5 years after publication, to allow for fixing mistakes like with Night of the Living Dead.)