It seems that Yahoo has a problem with moribund accounts- many people had a Yahoo ID 10-20 years ago, and then abandoned it. If these accounts are not deleted

It seems that Yahoo has a problem with moribund accounts- many people had a Yahoo ID 10-20 years ago, and then abandoned it. If these accounts are not deleted (and there are a bunch of organisational reasons not to), then the MD5 hash has to be kept around somewhere, until the user re-enters a password and a better hash is generated.