When the 3 remaining Beatles did the Anthology project, they finished two songs John Lennon had made demo taps for: "Free as a Bird" and "Real Love". But they left a track behind because they couldn't get a clean track of John's voice: "Now and Then".
After Peter Jackson's Get Back documentary, they realized the technology, now backed with AI, had advanced and they decided to release Now and Then as the last Beatles song, with contributions from all 4:
There's a nice short film about the making of:
I admit it made me a bit tear-y.
On a tangent: for the first time I looked up the lyrics to "Maxwell's Silver Hammer". What the hell McCartney???
I didn't really encounter the Beatles until college, and even then it was more of the "Past Masters" compilations, so I've always been a bit behind on their later stuff... so I assumed "Maxwell's Silver Hammer" was, like, one of those quirky silly songs like "Octopus's Garden" or "Yellow Submarine", not a tale of a hammer wielding serial killer!