It seems that kernel 5.9 (currently 5.9.11-3) introduced a bug that makes display brightness adjustment not work on my Dell XPS 15 9575.
Pressing the keys, the brightness slider moves, but the backlight level does not change - with the exception of 0% turns it off anything >0 returns it to the setting it booted at (50.1…%).
After resuming from sleep, it gets stuck at 1% (very dim).
This problem is not present when booting with the 5.8 kernel. (5.8.18-1)
/sys/class/backlight = intel_backlight
Display driver = video-linux (HD Graphics 630)