extant has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is extinct.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
still in existence; not extinct or destroyed or lost
“extant manuscripts”
Synonyms, antonyms and the broader and narrower relations for extant come from Princeton University’s WordNet. Full sources and licences.