Block a built-in keyboard in macOS 10.15.6
I've checked Karabinier once again and I found a solution that will block build-in keyboard without messing QMK's OSM.

Disable a built-in keyboard when external is plugged. For me it's Planck EZ.

Be sure to also uncheck events modification in the &quot;basic configuration&quot; tab for the same keyboard.


That configuration seems to work for me.