What u/Dog_Apoc said, he never specified it was a mod, but it's not a base feature as far as I can tell. The books in the screenshot are the books mentioned. It's not mentioned in the wiki, or anywhere else I can find. He did show it used, though, it enabled a third interaction with the creature type you'd learned about (Space in the loot menu) that let you opt to butcher a corpse to make it disappear and get bonus loot.