Reduced functionality refers to cards that rely on other cards (some of which are Forbidden) so heavily that their usefulness is greatly reduced. These cards are often specially-designed to take out a specific card, and only that card, but are rendered useless in the Advanced and OCG formats, as the cards they are designed to go against are banned. Even in the Traditional format, or before the cards were placed on the Forbidden list, many duelists don't consider them to be very good, as they only work against a single, solitary card, and it could be considered a waste of resources to add them in for precisely that reason, in lieu of other, more universal and generic negation or removal cards.
Cards with a reduced functionality still have some kind of (marginal) use in a Deck, even if their primary use is no longer valid. These are usually Monster Cards with an effect that specifically targets one single card, but their nature as monsters allows them to be used as such, while ignoring their (now useless) effect. They can also be cards primarily meant to be used in a certain theme (such as "Crystal Beast" or "Alien"), but can still be used outside of their intended Decks, if barely.
- "Elemental HERO Neo Bubbleman" (relies on "Metamorphosis")
- "Avatar of the Pot" (relies on "Pot of Greed")
- "Spirit of the Pot of Greed" (relies on "Pot of Greed")
- "Guardian Elma" (relies on "Butterfly Dagger - Elma")
- All baby "Dragon Rulers": "Burner", "Reactan", "Stream", and "Lightning" (rely on "Blaster", "Redox", "Tidal", and "Tempest", respectively)
- "Phoenixian Seed" (relies on "Phoenixian Cluster Amaryllis")