Could it be possible to be poisoned by the metal on a permanent retainer after 15 years of wearing it?

Highly unlikely: The metal in your retainer is likely made from stainless steel and so little if any gets bleached/worndown or get absorbed into your body to cause trouble. Many people have joint replacement, steel rods/plates/screws in many area of the body and it is considered inert to your body. So, if you are having some health issues/concern, consult doc for care but i would not blame the retainer. Good luck.

Most unlikely: Your retainer is made of a high grade of stainless steel(assuming usa fda standards). There are many other replacement body parts/joints made the same materials that are considered safe and effective. The glue used to adhere to your teeth is under the same set of rules.