What is undefined in this case?

In this case we took something that was an error and made it do something useful. Generally we consider that to be an additive change and we do that all the time.

Taking something that is undefined and making it defined as something new is another place to play but you have to worry about good old Hyrums Law

I mentioned the destructuring but maybe that one is not correct. As another example, clojure.set functions assume that their inputs are sets, and passing in other types is “undefined” (tho they have specific behavior that people might rely on).

Does that ever influence your decisions when it comes to defining or not defining behavior?

Of course