The Saprovore mutation allows your character to consume meat chunks that other characters would find inedible. This includes rotten, mutation-inducing, and rot-inducing chunks, all of which are treated as clean. Ghouls begin play with this mutation.
The exact effects of saprovore have changed dramatically over the course of DCSS' development. Prior to 0.10, eating contaminated chunks would occasionally inflict sickness, and rotten chunks were completely inedible. Having ranks of saprovore would make rotten chunks edible (though still potentially sickening) and would reduce the odds of getting sick from contaminated flesh. Any chunk could be eaten safely at rank 3, but only ghouls were able to reach that rank.
In 0.13, the nausea status effect was introduced. This was an intermediate status effect that would kick in before sickness, preventing the character from eating further until the status effect ended (though starving or near starving characters could force themselves to eat at a significant penalty and with a chance of advancing their status to sick). Saprovore functioned as before, except that it helped prevent nausea rather than sickness.
In 0.15, in an attempt to simplify DCSS' food system, all characters with Saprovore except for ghouls and trolls had the trait removed. This included hill orcs (rank 1), ogres (rank 1), demonspawn with the Foul Stench demonspawn mutation (rank 1), and kobolds (rank 2). It also removed the concept of contaminated chunks.
In 0.16, rotten chunks were removed entirely.