It depends on how you view 2D->3D.

If you're thinking of a side scroller like the original Super Mario, Y is up/down and X is left/right making the new dimention Z being forward/backward.

However if you think of 2D space like the first LoZ, then Y is North/South and X is East/West making Z up/down

Almost the entirety of computer graphics uses the z coordinate for depth afaik.

Even Minecraft does it.

As someone who looks at this from a GIS / cartographic view, the top option being possible is horrific to me.

https://help.autodesk.com/view/MAYAUL/2024/ENU/?guid=GUID-FDC58F4E-63B9-4012-B232-5F2FBAC5EAC9

Y-up and Z-up

In animation and visual effects, the tradition is to use Y as the “up” or elevation axis, with X and Z as the “ground” axes. However, some other industries traditionally use Z as the up axis and X and Y as the ground axes.

Surely you mean the bottom one