I can't find any image of a C64 keyboard PCB with diodes, so I don't think any C64 keyboard is NKRO.
Therefore, the layout of the matrix would limit which keys that could be registered when pressed together. A chart of the matrix was easy to find on Google Image search.
WAS and left shift are on the same row. A and D are in the same column.
Either of A or D would get blocked if you would press both of them while any of the W, S or Shift are pressed. But A and D are opposite directions in WASD so you wouldn't want to press those together anyway.
So, I see no limitation in the C64's hardware for WASD + shift. Unless the Keyrah's microcontroller is programmed like crap then I think it should work.