Fun fact about the creation of West Virginia is that it was arguably unconstitutional because the Constitution does not permit the creation of a new state from within the jurisdiction of another state. In order to justify the legality of that action, it has to be accepted that the lawful government of VA (as opposed to those in rebellion claiming to be legitimate) actually ceded those counties to the USA, thus making it territory of the USA and possible to create a new state from the territory.

The proposed State of Jefferson would however, be constitutional because it would be the creation of a new state from parts of already existing states, assuming of course the legislatures and Congress all approve the action.