Temporary workaround: chown/chmod
/sys/class/backlight/brightness to give your user write access.
Are you sure? I don't remember logind ever changing permissions on /sys/class/backlight/brightness One of the reasons why brightnessctl has these udev rules
No, I assumed. Didn't feel like digging too much into it at the time.
However, now that you point it out, the reason is probably brightnessctl's udev rules (which I have always had installed). These have been removed on the 4th of April from the Arch PKGBUILD: https://git.archlinux.org/svntogit/community.git/commit/trunk?h=packages/brightnessctl&id=f8ca4aeff714e76540cb863d1eff26145034b055
Instead, it was changed to use logind's weird SetBrightness interface. This, the fault is more indirect.