Yes, unfortunately it is considered unethical. My son did it during practice with his coach and I have never seen that man so livid. He explained to him that you can do that but it somehow discredits your character overall and he showed him a slow spin serve to replace the under arm serve.
I made 2 mistakes in my early 20s that I deeply regret. First: making people who’ve hurt me believe that we’re cool because honestly, we are not. I forgave you to free myself, but the North remembers. Second: wasting time being sad about how people have treated me. Never again.