In the chapter about upper-bounded wildcards (Chapter 14 Generics and Collections) this is not entirely true:
"Something interesting happens when we work with upper bounds or unbounded wildcards. The list becomes logically immutable and therefore cannot be modified. Technically, you can remove elements from the list, but the exam won't ask about this."
Boyarsky, Jeanne; Selikoff, Scott. OCP Oracle Certified Professional Java SE 11 Developer Complete Study Guide (p. 651). Wiley. Kindle Edition.
Technically, you could add a null value making it mutable in both ways (you can both add and remove items).