Affordance is a used extensively in the usability field and is a fundamental part of the vocabulary. Don Norman, Jakob Nielsen and Alan Cooper all use it quite a bit in their writings, so anyone who has background studying the literature of the field will be familiar with it. Not knowing it is something of a shibboleth that suggests one isn't familiar with the basics of the profession. It would be difficult to be in the usability field for long without having read works by one or more of the three practitioners mentioned, or having met them at conferences, or run into discussions with others about their work. The term is also used by many other designers as well, I just mention those three since they are pretty big names and I happen to see books by Norman and Cooper on the shelf directly above my monitor.

Yes, usability testing confirms this, which is why you do it. Put key functions in hidden controls and then ask people to perform tasks that require discovering the controls. Most won't find them. In another comment here I mention my experiences with usability testing and finding that the much more well known paradigm of context menus is used or thought of by extremely few users. Advanced users will use it a lot, especially after the first time they look for context menus and find they are there and robustly designed in a given app, but non-advanced users, which is the vast majority, will never look there. In working with customers, some find the existence of features in context menus only after many years of work with a given program. One absolutely can not depend on users to assume that needed functions can be found in invisible items.