It’s kind of a niche requirement but very important for someone like me who runs 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.