I’ve worked in environments where USB is 100% disabled, including HID.

Today, I believe but can’t find an affirmative document, that some motherboards have an option to hijack USB HID mouse & keyboards and present them to the OS as something other than USB.

"Legacy USB support" is exactly this, uses SMM/ring -2 software routine to pass data from USB devices into emulated legacy 8042 keyboard & mouse.

a) gamers (supposedly lower latency and better n-key-rollover, realistically not really relevant)

b) servers for KVMs/crash carts/... that are still VGA + PS/2