I can confirm, that
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nvidia.NVreg_RegistryDwords=EnableBrightnessControl=1"
on Ubuntu 22.04 and GeForce RTX 3600 mobile with the 470.XX.XX driver works on a Legion 7 (2021) (16ACHg6) for controlling the backlight - while it does not with 515.XX or 520.56.06 (with 520.56.06 there is even one more regression: the backlight is completely off after sleep - so the display is basically off. As you cannot control it, you have to force-reboot…).
There are some ACPI errors regarding backlight:
[ 5.911249] ACPI BIOS Error (bug): Could not resolve symbol [\_SB.PCI0.GP17.VGA.LCD._BCM.AFN7], AE_NOT_FOUND (20211217/psargs-330)
[ 5.911350] Initialized Local Variables for Method [_BCM]:
[ 5.911350] Local0: 0000000013343bf7 <Obj> Integer 00000000000000FF
[ 5.911354] Local1: 000000002274e16a <Obj> Integer 0000000000000000
[ 5.911357] Initialized Arguments for Method [_BCM]: (1 arguments defined for method invocation)
[ 5.911358] Arg0: 00000000510381c2 <Obj> Integer 0000000000000064
[ 5.911363] ACPI Error: Aborting method \_SB.PCI0.GP17.VGA.LCD._BCM due to previous error (AE_NOT_FOUND) (20211217/psparse-529)
[ 5.911494] ACPI: \_SB_.PCI0.GP17.VGA_.LCD_: _BCM evaluation failed
Interestingly: on a Legion 5 (2021) (15ACH6H) with otherwise the same software setup and BIOS revision (gkcn57ww), the backlight control works with 515.XX (didn’t test with 520).
In both cases the graphics are set to “discrete” mode, so the NVIDIA card is the only one in the system.