People usually change their username due to privacy reasons and making such announcement would defeat their purpose.
If you posted a feedback for a user and the user changed his/her username, it shouldn't be difficult to find out who he/she is.
Yea, making such an announcement for change of username might automatically translate to defeating the purpose for which the change was made.
It's only applicable for users who posted a few and become inactive after the change. Not those prominent users, even there's no announcement made people will notice it eventually just a day or two. Remember old quoted posts remain the old username in the post too.