MBP Touch Bar Mirror Displays Keyboard Shorcut
The new shortcut is Command-Dim Brightness. It even makes sense in the following way: 

Assuming that you take advantage of the touch bar, and it displays the system controls instead of the function keys then the hand gesture is exactly the same as before when you would have pressed Command-F1 since the Dim Brightness button is where F1 would be: 



The annoying thing is that if you have an application-aware touch bar configuration, then you have to bring up the system controls touch bar configuration first. 

This is why, in some cases, the Display Menu (in the App Store) which adds a status item in your menu bar (and includes a Mirror Screen choice) is a good option.