Earlier quoted context omitted.
Yes, UML diagrams. One should be able to read and understand them.
How hard is that to learn though? That seems like a fraction of the cost it takes you to learn OOP/SOLID/etc.
UML is pretty straight forward, and I rarely saw it being used in companies because of many reasons (spaghetti monster being the one :D), but I mentioned it because I'd expect that one showed now how to read them, since you will find them in literature (if you read any book on design patterns they are there).