This is a common urban myth, but is untrue. There is no special part of the Texas state constitution that makes any mention of the ability to leave the USA. Nor is there anything in the USA constitution which would allow for this. It most likely comes from a clause in the document annexing Texas to the United States which allows for the state to break into multiple states. However this probably doesn’t bypass the normal process of granting statehood. So if for all practical reasons not a thing.