Why "Automatically adapt to environment" (ambient light sensor auto-brightness) doesn't appear on X11 in KDE Plasma 6.6+
Plasma 6.6 shipped automatic screen brightness adjustment driven by an ambient light sensor (ALS). The feature requires iio-sensor-proxy to be installed (it's an optional dependency of qt6-sensors and not pulled in automatically on Arch). Once installed and the Plasma session restarted, a checkbox labelled "Automatically adapt to environment" is supposed to appear in the display brightness controls.
If you're on X11, it won't appear — even if iio-sensor-proxy is installed, the service is running, and monitor-sensor confirms the sensor is working correctly. This is not a configuration problem. The feature was never implemented for X11.