I do remember something about old plastics from those time periods decomposing(?) over time, Shrinking, color change, And even putting off some kind of fume(?) or something that will corrode metal parts. Pocket and Wrist watch Plastic Crystals from that time period are a prime example of this! Not sure if the same type of plastics were used in pinball or not. Just saw the thread and kind of remembered something about it from working on watches as a hobby in the past.

I would imagine that it applies to most plastic of the time? And UV and heat from the lights, mentioned already.... Maybe it all plays into what happened. Even on my couple of projects from the 80's and 90's , the plastics are brittle and break pretty easy.

All I remember is something about the reaction going on in the plastic and age has some kind of reaction with metal. Or will corrode metal.
Too many pins might have the direction with that on the Nitrate and Acetate. I have no clue... I'll try to find an article I read on it. And post a link here if I find it. Basically it decomposes or starts reacting or something. I just know an old plastic crystal on a pocket watch will destroy and corrode the metal in the watch from whatever happens to the plastic. It shrinks, starts turning color, etc.....

ok..I'm just rambling...I'll go try to find the article...

Here's a link that gives a really good description of what happens. This isn't the one I was looking for but will explain some until I find the other one. And I have no clue what type of plastic was used in pinballs. But the descriptions of what happens seem to match your pictures.

https://www.nps.gov/museum/publications/conserveogram/08-04.pdf

I have a Mr and Mrs pacman. I finally got it working with a ton of help from an awesome member from pinside. Months of him helping me in PM's and threads. All the plastic drop targets looked great!! I took the drop targets apart, cleaned, touched up the paint on them, made them look almost new!!! After all of this? I broke most of them in one or two games from them being so brittle.