To further elaborate on this, Araya has all of the classic signs of someone who has strained his vocal cords. His abrupt loss of vocal range and shift to less aggressive vocals make it pretty obvious. You can also hear a certain roughness and uneven quality on South of Heaven that is indicative of vocal strain.
He used to utilize one of the most common techniques that causes vocal cord damage, which is using what is obviously self-taught vocal belting to hit high notes. Generally, this is an inadvisable technique but many singers do it anyway. I think that Chris Cornell from Soundgarden is the most famous example of someone who did this and suffered vocal damage. He used the same technique.