I do not even think DPMS support can work under UEFI for 1 very simple reason. The interface that is offered by the video card is stripped.
Deep under the bonnet there is a mini VESA BIOS providing SOME of the INT10 support under a UEFI boot. The INT 10 interface is the BIOS int10 used to talk to the video card in gone mostly. In UEFI boot you only have a frame buffer to write to its video data. Hence the reason the DPMS will most likely not work. I am almost certain off this.
You are spot on about "if the BIOS supports" these buttons. Most UEFI firmwares do not support these buttons at all. The basic trend of UEFI is an as stripped down as possible OS loader compared to a classic BIOS. The OS needs to deal with everything.
* Switching to an external beamer on your laptop. The button does not do anything. Why ? Because the UEFI GOP has no way of switching the HDMI port to for example an external port. It only works when Windows or Linux has started at the OS can make a call to the video driver to switch the HDMI video output port.
* Dimming the screen or switching off. Sadly with UEFI yet again game over. I would LOVE to have the ability to change the brightness of the screen.
The video driver MOSTLY deals with this. However this MIGHT somehow be able to made work on ArcaOS:
https://wiki.archlinux.org/title/backlightThe problem is I do not know how much work this is and how many machine support this route. Being able to reduce the brightness of your backlight can help increase the uptime of your laptop while running on battery power.
* The only thing that could be made to work "easy" volume up volume down buttons and do something with the UNIAUD mixer.
Roderick