Obviously mutability is not an issue in OOP alone. Note, however, how I said, “how normal is it that our methods mutate instance variables?”. A well-skilled programmer can always choose to not use the footgun a language hands them. But those are not the only people to consider.

As for structs, you could qualify the pointee of a pointer argument as `const` but this is as rare in C as the equivalent is in OOP languages.