We have these little turns of phrase to represent design concepts. One such phrase going back to Dredmor is “put it in the Hat Pack”. I consider this a form of subtractive design.
So: The Hat Pack is an expansion pack for the game you’re working on which is shrouded in the mists of the future yet-to-be. (And as the name suggests, the content is a bunch of useless hats.) You put a feature or idea into the Hat Pack when you need to kill it, cut it, remove it from the game; but you don’t have the heart to kill it. So you quietly put it in the Hat Pack and shove it at the back of a closet to be slowly forgot.
(Or not. I want to say that features have legitimately been revived from the Hat Pack, but the notable example that comes to mind is the facial hair equipment slot for Dredmor that involved a bunch of false beards and mustaches to enhance your powers. Never happened.)