Programmers seem to be using the screen as the flat reference.

In screen coordinates, the origin is the top left corner of the screen, and the Y-axis increases towards the bottom of the screen. So Y still isn’t “up”

You looked down at the math book on your desk that showed the X-Y graph on the page, and the Y axis extended forward, away from you. Z was “up”.

Z-up right handed makes the most sense