I know plastics are subjected to accelerated aging tests like heat/cool cycles, UV light, soaking in hot water, etc. Do any manufacturers have samples set aside that are many decades old at this point?
Some gun powder manufacturers store their “waterproof” powder under water and periodically dry out a sample and test it. Some of those sample are probably 100 years old by now and they still ignite to spec.
That is going to vary a lot by manufacturer. I used to work for a company that molded nylon in a variety of items generally for electrical use, especially for stuff going into panels. There was a small collection of parts dating back as far as the 70s.
I’m a retired manufacturing engineer. I didn’t work in plastic manufacturing but I’m very certain that keeping samples would be common practice for any reasonably sized manufacturer.