Xbacklight is used to adjust the backlight brightness where supported. It uses the RandR extension to find all outputs on the X server supporting backlight brightness control and changes them all in the same way.
All questions regarding this software should be directed at the Xorg mailing list:
https://lists.x.org/mailman/listinfo/xorg
The master development code repository can be found at:
https://gitlab.freedesktop.org/xorg/app/xbacklight
Please submit bug reports and requests to merge patches there.
For patch submission instructions, see:
https://www.x.org/wiki/Development/Documentation/SubmittingPatches