I saw Judy Garland perform in New York around Christmas 1967 when I was in the Army stationed at Fort Dix, New Jersey.. I believe the film covers her life in London, a year after her New York Christmas concerts. Judy's voice was adequate (just barely) in late 1967 because she still had enough vocal power to surprise. I remember she make "Get My Regards to Broadway" exciting -- not as easy task.
I suspect her voice had not improved much in London in 1969.