I can’t imagine this is necessary in order to clean up old films in general?
Blu-rays of old films aren’t made from projection prints because projection prints, in most cases, don’t have enough fine detail to make complete use of 1080p resolution. People like all the extra detail Blu-ray had the capacity to reveal, and 35mm negatives give us the ability to provide all that and more, so I don’t really blame anyone for doing what they do in this respect.
But if you go back to the negative or something like it, you’ll usually have to redo the color grade, because the colors on the negative frequently look nothing like the film as it was originally projected. And usually there’s some other cleanup people do as well. “Grain management” is part of the modern restoration workflow of any film-based movie, but what the grain management entails can vary. Degrain/regrain is one of the blunter tools in that toolkit, and all I can say for sure is that it’s used a lot, and it was used on the 2011 Blu-rays.