I learned today that StackOverflow (and company) have responsive support persons who respond via e-mail, that will help you recover and merge your cookie-based StackExchange accounts if you've lost access and there is sufficient evidence that they belong to you. (I thought I had registered the account under the myid.net provider (who is now defunct?) but no.)
That has to cost a lot of money. With the dozens of ways you can authenticate with StackExchange, I don't know why I was surprised to learn I had previously created an account without actually creating an account, but it was so.
Also, anyone who reads Korean can tell me if this says "sorry we're closed"?