I don't use this, but as an example: I like to use a US Intl keyboard layout for programming but as I am German I need äöüß a lot - so I've used a custom keyma

I don't use this, but as an example: I like to use a US Intl keyboard layout for programming but as I am German I need äöüß a lot - so I've used a custom keymap for caps-aous for years (and a couple other things). Yes, you can get most OSes to somehow do this, but just doing it at firmware level would fix SO MANY PROBLEMS.