Thanks for the question, I'm sure a lot of other golfers have similar questions. I hope that at some point we have a number we can reference (neutral) and you can quickly see if you are steeper or shallower than that. I think the challenge in writing the formula for this is the 3D nature of the swing. Here's a video going around of Jon Rahm. You'll see that he strikes the ball, and then the ground a couple of frames later. He barely takes any divot.
How is that relevant?
The low point would influence the shallow/steep ideal as much as the angle of the shaft.
In general, in transition, amateurs are steep in shaft pitch, but narrower in width than pros. The picture you posted is a good example of this. The angle is more vertical, but there is a good chance that the location of the club is further behind the ball than a pro at the same location. So neutral would depend on a few factors.
The one factor we haven't discussed is intended ball flight. A draw vs a fade would probably have different "neutral" numbers. And if you move the low point forward or backward, that would also likely change the neutral value. I would say neutral is close to the shaft plane (DTL), but also a certain width from the golf ball (FO).
What I currently do is look at a given position and add it up in my head. If the club is close to on plane, but the body movements are mostly shallow, then the arm motion must be steep (and vice versa). Or another way, from any given position, would a good swing require the addition of more steeps or shallows? That's a little more detailed than "I know it when I see it" but I recognize that it's not a clearly defined answer either.
So in the example of the picture you posted, the club is obviously more vertical than the shaft plane (so it's steep in that sense) but we can't see the width unless we also look at the face on. How steep it is would be a cool number to get, but to be useful, you'd have to break it down into each piece. In the example you posted, it appears that golfer is steep because of the thoracic spine and shoulder blade motion, but it could be some shoulder motion as well as a lack of depth (shallowness) at the top of the swing.