It's a different kind of example but The Other Side of the Wind by Orson Welles began filming in 1970. It only released in 2018, so 48 years. Welles never made a complete cut but he did edit quite a bit of it together and the rest was compiled together from the 96 total hours of footage Welles left behind, with his notes and some creativity. A few simple things were added but it's still 99% footage Welles shot in the 70s.

It's a very weird and experimental film, really cool cast, and I'm not sure if it is good or bad but the history of it made it worth watching. There's also a good documentary about the production, They'll Love Me When I'm Dead.