To add to this: the keyboard matrix scan controller on laptops is usually located inside the Super I/O chip and connected to the CPU/southbridge via LPC or eSPI. Since LPC/eSPI peripherals are mapped to I/O ports, implementing an 8042 controller means that the keyboard can work even without setting up a much more complicated USB host stack.

On laptops it's usually not SuperIO, but EC (Embedded Controller), the same chip that handles battery, power, etc. and is usually connected over LPC to "present" the keyboard as PS/2 compatible input (some new ones are pure USB though, like my current laptop)

Some devices have SPI interfaces for HID though

Wow, thank you to share this website. It is so cool. I never saw it before. New Wiki rabbit hole to explore!

That's a great site. Finally I think I understand WTF Lenovo mean by "IPMI over Keyboard Controller Style (KCS) Access"