Known? I don’t know about that, but I do know that if negatives aren’t stored properly they tend to discolour, especially if there are multiple, unprotected negatives stored on top of eachother. Not using the correct materials to protect them (a brown envelope for example) also influences this: Chemicals from the envelope react with the chemicals in/on the film.
There’s also the film itself. Although film is rinsed clean of chemicals when the processing is done, this doesn’t succeed 100%, especially if a 1-hour-service was used to develop them back when. And thus film has the tendency to keep developing, albeit at a very low level. This will be noticeable over a long time period though.
If you are talking about cleaning the physical film itself: I’m not entirely sure, but I don’t think you can. Some of the chemical processes might not be reversible. Dirt and other stuff that has gathered over time on top of the film might be removed, but I would advise you to ask a professional about that. Some of the dirt will have interacted with the top layer of the film.
You can pull these into a editor, one that has a good masking ability, and do a lot of damage control. There probably will not be a one-profile-fits-all solution. So it depends a bit on how much time you want to spend and how important these images are.
EDIT: I had a digital go at this image, just to give you an idea. This took me less then 15 minutes to do, so imagine what you can do if you have a serious go at this:
(58.7 KB) darktable 3.4.1