That is completely wrong. If you go back to the 1787-88 debates in the New York, Rhode Island, Virginia, and North Carolina legislatures, you will see many examples of secession discussions. There were several efforts to codify, modify, and place limits on states involved in seceding.
All of these efforts prove the opposite of the statement you made. State legislatures would not make secession illegal, and thus neither did the Constitution.
We do know that 74 years later, that the United States Congress judged secession to be legal....unilateral or not.
12/12/1860 On this date the United States House of Representatives proposed the following Constitutional Amendment:
”Whenever a convention of delegates, chosen in any State by the people thereof, under the recommendation of its legislature, shall rescind and annul its ratification of this Constitution, the President shall nominate and, by and with the advice and consent of the Senate, shall appoint commissioners, not exceeding three, to confer with the duly appointed agents of such State, and agree upon the disposition of the public property and territory belonging to the United States lying within such State, and upon the proportion of the public debt to be assumed and paid by such State; and if the President shall approve the settlement agreed upon by the commissioners, he shall thereupon transmit the same to the Senate; and upon the ratification thereof by two-thirds of the senators present, he shall forthwith issue his proclamation declaring the assent of the United States to the withdrawal of such State from the Union.”.
Since secession was not specifically prohibited by the Constitution, Congress attempted to pass an amendment requiring approval.
It passed the House 154-14. It soon failed as well as at least two more efforts to legislate rules regarding secession.
Efforts were made but unilateral secession was never prohibited by law.
Yes, they did. And no they didn't include them in the final document. None of them accepted the notion of unilateral secession or dissolution “at pleasure”.
We do know that 74 years later, that the United States Congress judged secession to be legal....unilateral or not.
In other words, they recognized that secession was not currently legal and proposed an amendment to the Constitution to make it so. The proposal did not survive the constitutional amendment process and thus is not law.