Can't say for certain the shelf life of rubber, but for sure its better on the shelf than sitting out in the sun, so it's already got a point for being protected from UV rays that would dry it out.
I replaced the bushings on my car with new ones just for the sake of having new ones on a 20 year old car, and when I removed the old ones they seemed perfectly fine and comparable to the new ones.
If the ones on the car still look intact and the rubber hasn't broken free of the shell or insert, then it's still good.
Similarly, engine mounts likely have the same rubber and I bought OEM ones from the dealership from their vintage catalog, and they've been working fine. No crumbling or cracks despite sitting on a shelf in a warehouse for years.
I replaced the bushings on my car with new ones just for the sake of having new ones on a 20 year old car, and when I removed the old ones they seemed perfectly fine and comparable to the new ones.
If the ones on the car still look intact and the rubber hasn't broken free of the shell or insert, then it's still good.
Similarly, engine mounts likely have the same rubber and I bought OEM ones from the dealership from their vintage catalog, and they've been working fine. No crumbling or cracks despite sitting on a shelf in a warehouse for years.