Isn't this just reflective of the fact babies with larger heads are less likely to die in childbirth now than they were in the past? Due to increasing use of c-sections?
Could be that plus better nutrition and overall better childcare.
I suspect that we probably reached the peak of that effect decades ago though. With increased environmental degradation (ubiquitous microplastics, more processed food in the diet, etc.) I suspect we’ve begun a long downward slide decades ago.