Throughout the Star Wars original trilogy, it was always assumed that Darth Vader's body was mostly mechanical. And with the release of Star Wars: Episode III - Revenge of the Sith, it was revealed that Anakin had indeed lost all his limbs before being placed into the iconic Vader suit. However, a subtle detail in Star Wars: Episode VI - Return of the Jedi reveals that Darth Vader was even more mechanical than first thought.
By the time of Return of the Jedi, Obi-Wan claims Vader is "more machine now than man, twisted and evil" -- referring to both his mind and body. And he's undoubtedly correct, as not only did Vader have robotic limbs, he pretty much depended on the suit for survival. So much so, that he eventually added even more mechanical parts to his body to stay alive and become stronger than ever.
Darth Vader Was Almost Entirely Machine
As Luke Skywalker and Palpatine have their final confrontation, Vader finally has to choose between his son and his Emperor. As Anakin returns once more, he lifts Palpatine into the air while Force-lightning strikes across his suit. It's during this that quick frames of Vader's skeleton can be seen, and by pausing it shows just how mechanical his body really was.
Alongside the robotic limbs, Vader's entire spine appears to be made of metal. And despite his head being organic, robotic joints can be seen going into his skull and acting as a new neck. Also, while Vader's torso isn't shown, it's assumed that his ribs would have to be mechanical as well, and possibly his entire skeleton. If this is the case, then almost all of Vader's body had become robotic, all except for his vital organs.
When Did Darth Vader's Body Get Replaced?
Fully cyborg characters are rare in Star Wars, but not completely unseen. The most notable cyborg is General Grievous, who had a beating heart and a brain, but not much else inside his powerful shell. As seen on The Book of Boba Fett, Fennec Shand has her entire stomach replaced with machinery to survive. And so having Vader be almost entirely robotic isn't too absurd. But as to how he got to this state, it's mostly explained in Star Wars comics.
The Darth Vader comics tell the story of Vader after finding out Luke was his son. To discover the truth, he investigates the death of Padmé to understand how his son was alive, much to the disapproval of Palpatine. As punishment for holding onto the past, Vader's master destroys much of his suit and leaves him for dead on the planet Mustafar, commanding him to survive without the use of the Force. After much pain and endurance, Vader manages to live and make his way back to Palpatine. And it was here that Vader was given a new suit and cybernetics, likely including a replacement spine.
To keep his strength and stay as Palpatine's apprentice, Darth Vader essentially became Frankenstein's monster. The brief shot of his skeleton adds even greater context to "more machine now than man," as he was close to becoming nothing more than a brain inside a metal suit. But despite this, Vader still managed to find some humanity within himself and save his son, resulting in one final act of kindness before his mechanical body was the death of him.