I have a close relative who does college admissions. I asked him what he does with a thank-you note. He wrote, “I keep it among the other thank-you notes I have. But that pile is small so they are special.” I asked whether he noted them in their files, and he said, “I note it in my head.”
So, does a thank-you note make a positive impression? Yes. Is it going to make an inadmissible kid admissible? Not likely. Is it memorable and a nice thing to do? Yes. Is there a downside? Can’t think of one.