The phrase "be extant" refers to something that exists or is currently present. Antonyms for this phrase would include "nonexistent," "absent," "vanishing," "disappearing," or "extinct." These words indicate that something is not currently present or has ceased to exist. Other antonyms could include "obsolete," "outdated," or "archaic," suggesting that something was once present or relevant but has since fallen out of use or relevance. It is important to note that antonyms for "be extant" may vary depending on the context in which the phrase is used, such as whether it refers to a physical object or an abstract concept.