It is described in the ShA as a minhag. Kitzur 131:6 says it is a mitzvah, not a chovah. And I heard in a shiur from R’ Rakeffet that Rav Soloveitchik did not go to the mikvah Erev Yom Kippur.
Aruch haShulchan says “nahagu kol yisrael.”
In any case, it is a minhag, not a chovah.