The Fifth Postulate is not "unproven" in the sense a mathematician would use. It is an axiom. Would you call the first four postulates "unproven"?

What Lobachevsky proved in 1829 was that this axiom was independent of the other four axioms. Riemann was considerably later, though he was more general, and there were people before Lobachevsky who essentially proved it but did not publish - see here.