They were on a deadline and they were unable to locate any materials for the last reel in time to make the premiere screnings. They decided to go with what they had and that included the last reel in cinecolor. AFTER that, they located a dubbed Italian dye transfer print that allowed them to recreate that final reel. It's been a complete restoration since about 1985.It doesn't say anything about any missing reels, and the Wikipedia's own page on lost and partially lost films says nothing about Becky Sharp being lost or incomplete.
I believe (but I would need to locate that really wonderful American Cinematographer article) that one of the three records was almost completely lost (I want to say yellow but I can't be 100%)
EDIT: Closest I could come was an excerpt on Questia. I had it wrong about which color record was missing. If anyone can find the full article, it was really quite good. There's every chance a copy of the magazine is still somewhere in my mom's house! LOL