That strikes me as... odd. Why would it disappear? Unless they wanted to nuke it completely because of possible visibility/account permissions based on the old closed beta team.frankenstein branch just got axed. It hadn't been touched since last November.
Which makes me think that Vesuvius might be further along than we suspect if they're about to create a new closed testing build that requires a new test group and they need to be confident of who has access to it.
Or, of course, they could be scrapping the whole thing and moving on to Civ7, because Civ6 is abandonware. Because doooooooom.
Wait... what if the leak is from a spin off game? BUM BUM BUM!