Hello there,
My username is WindraX on Steam and I bought the game yesterday, on steam, and like the others I do not have a "DLC" tab in the properties window of the "Divinity: Original Sin 2" game.
I would like to note that I've noticed that on the "receipt" for my purchase of the game in steam (under my account details, in my purchase history) the game is labelled as "Divinity: Original Sin 2 [b][u]Launch[/u][/b]".
Could it be that you, or Steam, has published the game under a different label on release, making your changes to the "availability of the DLC" unsynchronized across all customers?
I bought the basic game on Steam yesterday, the 15'th of September 2017, for a price of 54.99 CAD$.
Thank you for fixing this as soon as possible and keep up the good work!