Are the contents of discussion between a University and a student private or public?
You're not obligated to tell them. Whether you tell them or not would be unlikely to have an impact on your future either. 

They're probably not going to track you down and find you, and would mostly use the information to improve recruitment in the future. 

Imagine if you told them you chose University X instead because you had family there and it was easier to secure housing. That's a sound reason, and unlikely to cause any sort of mark in the off chance they were tracking you.

I would try to avoid saying anything bad about the school, though.