In Edit Mode, are 2D curve points/nodes clamped to Z=0?
Isn’t it because it is '2’D? I believe in order to change z value, you have to switch to 3D
A curve could still be FLAT, ie 2d or planar, and not have any of it’s points at Z=0. IOW, you could rotate a 2d curve just like a plane.
But I think to make their math easier, the devs by decree just said "all 2D Curve points will share the same Z coordinate in Edit Mode, and then layer the Object Transform on top of that. Edit Mode seems to be in Local Coordinate space (for 2D Curves). So, restricting points to Z=0 simplifies things. (I suspect this is just an ancient hangover.)
To be clear: when you select any point on a 2d curve in Edit mode, the N panel always shows its Z coordinate to be zero.
The coordinates of the control points of a curve are local (curve) space and if it is 2D then z=0… always… ever… nothing ancient… just fact…