The world is nothing but edge cases. Expressing anger requires cooperation. If someone doesn’t want to listen, and you express anger at them anyway, is it not abuse?
And it's not just anger. If I want to express verbal affection towards someone and they don't want to hear it, we often call it abuse. But if they want me to suppress verbal affection towards them and I don't want to, we celebrate it as them having boundaries.
And I think sometimes being forced to not say something can hurt as much as being forced to hear something, yet we don't seem to see how it harms people.
But you can also process anger solo with the right tools, or with a good therapist that doesn't shame anger. It doesn't always have to involve another person.