Almost Nothing Has Immunity Bees!

Okay, here’s the logic:

1.) Elves live forever unless killed

2.) Elves cannot grow back limbs like starfish (c.f. maedhros)

3.) Elves, due to their preternatural endurance, are very likely to survive having limbs hacked off (c.f. maedhros again)

4.) Elves, especially in the First and Second Ages, spend a lot of time trying to stab each other and/or fighting things that want to rip them to shreds (orcs, werewolves, etc.)

Rinse and repeat long enough, and by the Third Age, most older elves would have several serious permanent injuries. Conveniently, elves also have a cultural predilection for a.) the healing arts and b.) magic smithery, which gives us an excellent fast-track to automail-type fantasy prostheses.

I drew Maglor and Galadriel, who have decent odds of rocking the most metal-per-hröa by the time of LotR.

Maglor has a vocal prosthesis (maybe for smoke inhalation during the War of Wrath, specially calibrated for singing-laments-upon-the-shore purposes); a cranial implant/helm (I like to think Elwing chucked something heavy at him); a gauntlet (to restore fine motor control to his Silmaril-chucking and harp-playing hand), and a prosthetic leg.

Galadriel has several implants and a prosthetic arm. The crown-with-circuitry is a helm designed to offset migraines (a common side effect of peering into things that are, once were, and have not yet come to pass.) It was made for her by Celeborn, who is her husband AND her mechanic, and is also why he gave her the nickname Galadriel.