I've been making a lot of USB Keyboards recently, and have been using a variety of controller boards. The older ones used ATmega328P microcontrollers on matrix board (as seen here as a ZX Spectrum Plus USB Keyboard)
ZX81 USB Keyboard).
Commodore 64 USB Keyboards
Acorn Electron USB Keyboards,
USB Keyboard Controller Kits are available from my Sell My Retro store: