There IS dimension constraint in SU!
Here's an image to show drafting assistant helping me select the plane in which my dimension exists.
sample1.png
I wouldn't be expecting to find it in LO if it weren't in SU. I can see from your pictures that you didn't pull the dimension away from the geometry being dimensioned. IRL you never put dimensions right on the line being measured; it's illegible in the field.
I'm not looking for this to be used in perspective, although the behavior should be consistent across the whole SU platform, regardless of what view you're in.
Please do add this as a feature request. There are a lot of people trying to use SUPro to make construction documentation, including me. It's almost never useful to reference the hypotenuse of anything.