You could use a combination of TOTP/google/facebook or some other side channel verification and use that to allow unlimited tries for a certain period of time t

You could use a combination of TOTP/google/facebook or some other side channel verification and use that to allow unlimited tries for a certain period of time to allow for more guesses? I'm thinking that for the most part people generally have/keep <5 email addresses (that they use often enough to log in with) so they should be able to just iterate through the list of emails that they've used over their lifetime and figure it out? I do wonder though -- if the hash secret gets out then I think we're right back to where we started... it would be easy to cross-reference leaded email DBs with the dumped DB and work backwards. I'm now a bit less sure this does much for use privacy against an even slightly motivated opponent (one who would almost certainly have access to at least one dump of previously-exposed emails)...