It’s kind of a niche requirement but very important for someone like me who has does presentations with keynote split on two screens to running demos that need to be mirrored.
It was always Cmd-F1 on any non-touchbar equipped Mac, but sadly that stopped working on the new Macs, even if you hit the fn key to make the function keys appear.
Step one :
In Keyboard Preferences, select Press Fn key to Expand Control Strip
Step two :
Hit Fn-Cmd and the decrease brightness button on the touchbar.
If you’re interested, here’s the post with full instructions on how to make the hardware version.