Under Settings>Configure Krita>Canvas Input Settings>Zoom Canvas, Key Combinations set to Relative Zoom Mode do not work. Only Mouse Buttons work with Relative Zoom Mode. I expect the canvas to zoom in when holding a key and moving the mouse/cursor upwards, and zoom out when holding the key and moving the mouse downwards, like how it behaves with Ctrl + Middle Button.
Yes, this is currently not possible. Setting the status to wish and confirmed.