Damage Diet is a Normal Trap Card.
Trap Cards are normally Set first and can be activated on a later turn, often in response to an action.
On the current TCG and OCG Forbidden & Limited lists tracked here, Damage Diet is Unlimited.
YGOPRODeck lists Damage Diet for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
A listed print of Damage Diet appears in Photon Shockwave (PHSW-EN065) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Damage Diet is about $0.03 on Cardmarket (prices change frequently).
