The full writeup (including seven hours of video) is at http://cowlark.com/2019-11-03-keyboard/, but the executive summary is:
- SMK spring over dome tactile linear switches (very nice) in a matrix layout --- sadly no diodes
- The 15-char LCD screen presents itself as a USB serial port so the computer can write text onto it
- PCB modification to remove the hard-wired typewriter-style shift key logic
- Several more PCB modifications to convert matrix keys to more modifier keys
- Based around a PSoC5LP development board
- The paper feed mechanism still works (although I did have to remove the thermal print head), and makes a great phone stand
- Quite a lot of terrible soldering