The diodeless matrix
Like most rubber dome keyboards, there are no anti-ghosting diodes. Instead, the matrix is arranged in a way that it is unlikely to be able to press any combination of keys that would cause ghosting, or blocking.
The matrix on the Raspberry Pi 400 keyboard is 8x16. 24 total io pins used. The connector is 26 wide, 2 are not used (H and J in my numbering).
This is for the ANSI layout keyboard. Someone with the ISO version will need to compare.
The order of the pins on the bottom of the PCB are not in the same order as the ribbon connector.
My original labeling using A through Z on the ribbon connector.
This matrix/keymapping has not yet been verified. I have tested the matrix here.