There are subtle ways to add extension points that most people won’t even notice.

For instance, putting related code into a data structure allows you to add more to it later without replumbing the whole system (as opposed to functions with five arguments).

With enough experience with refactoring, just choosing a structure that’s easy to refactor can serve as an extension point. (I have even tricked people into implementing a feature the way I had it in my head by writing the intermediate version in such a way to suggest a simple change for part 2.)