It seems Princeton is also GRE optional - math.princeton.edu/graduate/admissions , same with MIT - math.mit.edu/academics/grad/admission/index.php, CAL - math.berkeley.edu/programs/graduate/admissions , Stanford - mathematics.stanford.edu/academicsgraduate-students/… . I suspect Harvard is doing/will do the same. So probably if you are just applying to top-6, you should be OK. Nevertheless, it is always smart to cast a wide net in your applications, especially giving the current pandemic and academia/job market effects.