I haven’t looked at this case in a while, so I probably should do that again. From what I can remember from previously, when I tested reading keyboard output with the IOKit library in MacOS, it’s indeed true that the keyboard doesn’t output anything for the FN toggle key. However, in order for the OS to actually detect the difference (which it does, as it must, to provide alternative functionality), the FN keys then must send a different set of values, so the OS can still detect that the FN toggle key is held down.

in order for the OS to actually detect the difference (which it does, as it must, to provide alternative functionality

That is true as state, but it is not true that all functionality is provided by the OS, for example Fn combinations that might alter functionality but occur totally via hardware in some computers are touch pad, wireless and screen toggles.

Right, that can be an issue. The post does mention a possibility for reversing the behaviour, and I was planning to open a question for that, too. I’m currently using an iMac keyboard which defaults to special functionality for the FN keys, then using the FN toggle to send keycodes to the application instead. That’s great on Macs, and I prefer it that way on my Macbook. However, this machine is replacing my Windows laptop I’ve used for game-playing, where the default has been to send key presses to the application, and the FN key used to toggle them into special functionality. I think that’s better for my usage of this particular computer. Hopefully, that’s possible to toggle, preferably in the GUI/terminal with a trace of it. Although I guess the UEFI settings suffice. Of course, that would be a less specialized version of the current question.

For actual keyboards (as a standalone hardware device) you won't find an option. I only encountered these BIOS options on laptops for the integrated keyboard.