The phrase "on shelf" implies that an item is stored, displayed or available for use. However, the opposite of this would be the absence of the item. Therefore, possible antonyms for "on shelf" could include words such as "out of stock," "unavailable," "missing," "obsolete," or "discontinued." The word "off shelf" could also be considered an antonym, indicating that the item has been removed from storage or taken out of circulation. Additionally, depending on the context in which the phrase is used, words such as "active," "in use," or "in circulation" may also serve as antonyms to "on shelf.