My child did mention legacy status in the Why essay but it was a humorous personal anecdote about a local eatery. He actually lived on campus for a few years as a little kid though…which he also mentioned in another essay in a very offhand way.
My younger son’s Why Chicago listed as one of his reasons he didn’t think he wanted to go to Chicago at first was that his uncle was a lawyer and he knew he had no interest in law. It allowed him to let them know that although he wasn’t a legacy he was familiar with the school, in a tongue in cheek way.
My DD applied ED to my DH’s Alma mater and got accepted. She also has four other aunts went to the same Ivy. However, in her essay, “Why ___”, she only talked about why she wants to go to that school, not why her relatives went. If you were to ask my DD in person, you have better sit down and get comfortable, because she will talk your ears off about the reason why she chose that school. She went to visit that school six times just to be sure. I think this kind of passion shined through in her essays, beyond the legacy hook.