But then both are just a kind of polygon, with an arbitrary number of sides of arbitrary length.

At which point you really want a variable length list of Sides, each with a length property.

Or you want a series of points with relative coordinates, making the sides implicit.

Or or or...

What I take from it is that there is no one true object model, there is no universally "correct" way of solving the problem. An evolving understanding of the problem leads to an evolving solution that places different priorities on different attributes.

Is there any value provided to the system by making Square a specialisation of Rectangle? Is there value in being able to express a Square without a redundant attribute value? What is the tradeoff? And so on.