I truly don’t think there is any specific cutoff. I just think the reality is that if you’re not in the top 10% of your class (in the majority of high schools), your academic record is probably not strong enough to be admitted to Brown. The remaining 7% probably accounts for athletes, legacy, and students from elite elite private schools that send half their kids to ivies.
The reason colleges want to see rank is because they want to see how your performance stacks up compared to other peers at your school – especially for lesser-known high schools, they want as much context as possible about your academic record.