I don't recall seeing a USB keyboard in a laptop (admittedly I don't do that for a living at any rate). Even backlights tend to be separate pwm lines. It also make sense as USB would require a controller on the keyboard side.

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"

Looks like it's again time for ThinkPad Linux user PSA:

  echo 255 > /sys/devices/platform/i8042/serio1/serio2/sensitivity

Until 5 or so years ago many laptop trackpads were simply PS2 mice (even with drivers installed).

PS/2 keyboard/mouse are interrupt driven which saves power on laptops. While USB ones are polling based which require controllers running all the time.

Good point. On a video recording system we had problems with dropped frames from the USB camera. When an external USB keyboard was connected the problems became rarer. Obviously the keyboard kept some component to enter deeper energy states, and when everything staid up all the time frames were no longer lost as commonly.

Surprising to me as a layperson in this field is that camera with 30 fps would allow entering a deeper state than a keyboards with a data rate orders of magnitude smaller.

May laptop's mouspad is connected over i2c. I wonder what the reason for that is besides saving a few bucks on cables.

In laptops, the EC is what usually connects to the keyboard and trackpad/pointing stick, and it communicates with the chipset via LPC (a variant of the old ISA bus). Many of them have a built-in 8042-compatible IP block for this. It looks exactly like a PS/2 keyboard and mouse to software, but the physical layer may no longer be present (some old laptops that support external PS/2 keyboard+mouse may have an actual PS/2 interface.)

If the keyboard stopped working (completely), I'd suspect a bug or random glitch in the EC firmware.