Other than the original 13, states didn't UNILATERALLY voluntarily join anything. They were admitted, and only with the consent of the rest of the states through a majority vote in both houses of Congress. Article IV specifically lays that out. If it takes approval of the other states to join then by implication is should take the approval of the other states to leave.
The Supreme Court has repeatdly held that each new state is admitted on an 'equal footing' with the original states, who DI ratify unilaterally. An even today, Congress votes to allow a state to join the union, but cannot vote for ratification - the people of the state decide for themselves - UNILATERALLY. Nothing implicit about it.