How is a frame designed for an easy step-thru? My question is very specific towards bikes which have a curved top-tube. How is it decided that a particular

height of the top tube above the ground for a set of people in a certain height range is good enough to allow easy step through? Is there any biomechanical aspect to it or is it simply decided based on a survey conducted among a large number of people in that height range?

Or is it decided purely on aesthetic reasons?

I am trying to come up with such a concept, but I am simply unable to find any strong basis for determining this particular parameter.
the step-through height must be no higher than the top of the pedal stroke, since the rider will need to have their foot that high to ride the bike.

The exception, obviously, is bikes for people who can't lift that high but can accept forced motion that high. But those are generally built for rehabilitation rather than primarily for transport, like the one below. Those do tend to be somewhat more flexible than is ideal, and often have quite low rider weight limits.

 (from CyclistChic)