Grades in rock climbing have a clear subjective component, even so, the peer review system is a well suited solution.

You can only review what you are able to climb, and can only propose one grade higher than the grade you are able to climb.

Current hardest route is "Silence", in Norway. With a grade of 9c, it was established by Adam Ondra in 2017. If you want to contest the grade, you need to do proof of work.