While it's nice that Michael reported about that relatively big but easy to miss feature, he got confused: That feature isn't new. It's in 4.10 since quite some time. The recent change is just that he fixed additional window decoration to work with it (initially only Oxygen supported it):
The Laptop decoration got such a button and generic support has been added to the Aurorae theme engine. Now almost all decorations shipped with KWin can provide this button.