This is a tricky one….with several different answers. Your can answer it as when was the last time a member of The Beatles was in the recording studios working on a track. One answer would be “I Me Mine” which was included on the Let It Be album. However, only Paul, George, and Ringo were present at the January 4, 1970 session as John was still on vacation at the time in Denmark. For me, the last Beatles session has to include all four members together in the same studio, which would be the August 20, 1969 session withd John, Paul, George, and Ringo, working on I Want You (She’s So Heavy) and the running order for the Abbey Road album.
A year after the recording, John Lennon had all but forgotten it, as this interview clip shows.