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.