Such a clause was not needed in the Texas Constitution or any other state’s constitution before the Civil War because it was commonly understood that the states could leave the union at will.
Not by everybody.
"An inference from the doctrine that a single state has a right to secede at will from the rest, is that the rest would have an equal right to secede from it; in other words, to turn it, against its will, out of its union with them. Such a doctrine would not, till of late, have been palatable anywhere, on nowhere less so than where it is not most contended for." - James Madison, 1832
"My opinion is, that a reservation of a right to withdraw...is a conditional ratification; that it does not make New York a member of the Union, and consequently that she could not be received on that plan. Compacts must be reciprocal - this principle would not in such a case be preserved. The Constitution requires an adoption in toto and forever. It has been so adopted by the other States. An adoption for a limited time would be as defective as an adoption of some articles only. In short, any condition whatever must vitiate the ratification...The idea of reserving a right to withdraw was started in Richmond, and considered as a conditional ratification which was itself abandoned as worse than a rejection." - James Madison, 1788
"But the ability and the motives disclosed in the Essays induce me to say in compliance with the wish expressed, that I do not consider the proceedings of Virginia in ’98-’99 as countenancing the doctrine that a state may at will secede from its Constitutional compact with the other States. A rightful secession requires the consent of the others, or an abuse of the compact, absolving the seceding party from the obligations imposed by it." - James Madison, 1832