It really comes down to looking at photographs made with modern digital lenses versus photographs made with antique lenses for film cameras. Do your own experiments: ideally, put the camera on a tripod, and do someone’s portrait with a modern lens and again with an antique lens with the same focal length at the same aperture. Then look at the pictures and consider which lens is giving the better rendering of the three-dimensionality of the face.
What you may first be aware of is the impression or feeling of the rendering: one looks somehow more ‘real’ or ‘natural’ than the other — closer to how you see the face with your eyes.
As I understand it, there are technical reasons for this difference, to do with the optics, with the behavious of types of glass, with microcontrast and so on. But what really matters is how the pictures look — draw your own conclusions from that.
But if you do want more technical detail, have a look at Yannick Khong’s articles:
The problem with modern optics
The flattening of modern lenses or the death of 3D pop
and he has a whole series of articles on his website about lenses.
HTH