How is a frame designed for an easy step-thru?
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)