Nope, never was, which is why it was such a constant and often used threat by most regions. The first states to use the threat were the New England states, upset over Jefferson's winning the Presidency and for decades after, and for reasons far less justifiable than the southern states had. The 1868 'ruling' wasn't explicitly about secession, it was about the appointed military governor of Texas wanting to steal some Texas state bonds that had matured for his own pocket, and of course the completely corrupt Chase SC gladly handed them to him, most likely for a suitable bribe, of course.