You can try to rehydrate but I bet they go to pieces, most rehydrates 1 LB salt to 1 gal water soak till plump, drain, and rehydrate overnight in a plastic bag in fridge overnight
I have heard some people are able to rehydrate skins 15+ years old. I have also seen skins that are dry tanned acid rot in less than a year. Just try one and see. Best of luck.
Years ago I used new method for capes, but I was always told they were only good for a year, unless they were frozen.
As has been mentioned, only one way to find out!