As a person with stutter, I personally wouldn’t mind a feedback but the way you worded your feedback is indeed very wrong and hurtful “it was pity you stuttered so badly I found it very difficult to understand you” What do you want him to do about it ? Magically stop stuttering? It takes enormous courage to give a presentation knowing you have stutter and then getting a feedback like that would just crush the person and destroy what confident they had. You could have worded your feedback differently please be more considerate of people’s emotions.