Win10 V.1511 and V.1607.
Example TaskBar Button - File Explorer

If you open an instance of File Explorer, then click the task bar button again, File explorer window minimizes. If you right click, a popup opens with Pinned, Frequent, and File Explorer. If you click on File Explorer, another instance of Explorer will open.

If you open an instance of File Explorer and then, hold down the "Shift" key and left click the File Explorer TaskBar button again, another instance of file explorer will open.

This works with most any application that can run in multiple instances. At some time in the past (i.e. probably Win7 era) I found a way to make individual Task bar buttons work as if the shift key was pressed (i.e. each Left Click would always open another instance of the app). After upgrading to Win10 and subsequently after loading the anniversary updates, I managed to lose that function. Since I've slept once or twice since setting this up, I no longer remember how I did it.

So to my question, Does anyone know of a setting or registry hack that will make a taskbar button function as if the shift key is pressed when it is left clicked?