That's actually an interesting question. SCOTUS kinda dodged answering it, though there's a 1970 decision that seems to imply they accept WV's validity. The ultimate question comes down to if states that left to form the Confederacy were still US states during that period, because that determines whether or not the clause in the Constitution about creating a state from territory held by another state applies. If Virginia was under the US Constitution during the Civil War then yeah, WV is an illegal state.