The Disney film first released after Walt Disney's death without his involvement was The Aristocats.
Walt Disney died on December 15, 1966, while The Jungle Book was still in production. Although The Jungle Book reached theaters in 1967, Walt had guided its development and remained personally involved before his death. The Aristocats, released in 1970, was the first Disney animated feature completed entirely after his passing.
The project had an earlier history: it began as a proposed live-action television story about Parisian cats before being reshaped as an animated feature. Walt approved the idea and reviewed early development, which is why descriptions sometimes call it his last approved project. However, he did not participate in the film's full production, animation, or release-era creative decisions.
The distinction explains why the answer is not The Jungle Book. That film was the last animated feature on which Walt personally worked, while The Aristocats was the first feature-length animated film made without his direct involvement. It premiered in Los Angeles on December 11, 1970, and received its general release later that month.