I only ever use the lowest keyboard brightness. I’m sure there are other people who only use the highest keyboard brightness. Some people will probably want their keyboards permanently lit or not lit, so they can use ‘fn-space’ for something else. Basically, I’m guessing that very few people actually use all four brightness settings, so why not allow users to disable some of them in the UEFI? Similar to how we are allowed control over the brightness of the power button?
I believe there is power button brightness control in the BIOS for the Framework 16 at least. Unfortunately, the BIOS options in the Framework 16 only allow you to choose between three brightness levels, none of which let you turn it off, although it is understandable why.
you have to dig through the qmk keyboard firmware.