Strictly speaking we don't know that. It may always turn up an extremely rare Y or mtdna variant which was thought to be extinct. Ötzi's mt like was thought to

Strictly speaking we don't know that. It may always turn up an extremely rare Y or mtdna variant which was thought to be extinct. Ötzi's mt like was thought to be extinct (Wikipedia page even still says so) but very recently a North African man took a full mtdna test and it turned out he had the same. That could happen with neanderthal variants too for all we know.