Disabled: Not currently available
Stomach Lining is currently disabled from the game. Assets for this exist within the game files, but will not be encountered through normal gameplay.
Stomach Lining is a natural block which isn't found anywhere, but it was probably meant to be found in the mission to defeat the Ruin. It's likely that at one point of development the player would have needed to travel through the Ruin's stomach, but the concept was cut.