How to disable the system feedback when using the “fn” button on a Touch Bar-equipped MBP

You can use my script until Apple adds a preference for the Touch Bar.

It simulates keyboard activity by sending the command for the fn-key every 60 seconds. You should not notice it while typing. Keep in mind, however, that this will prevent display sleep / system sleep as well.

Copy and paste the following code into the "Script Editor" application on your MacBook Pro with Touch Bar:

(*
1. Save as an Application: Script Editor > File > Export… > File Format: Application
2. Check "Stay open after run handler"
3. Run the app or add it to your login items: System Preferences > Users & Groups > User > Login Items > Press the "+" button

# http://apple.stackexchange.com/questions/269174/disable-the-sleep-function-of-the-macbook-touch-bar
*)

on idle
    tell application "System Events" to keystroke (key code 63) -- press fn key
    delay 60
end idle

Please let us know if this solves your issue.

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.