There is "kind of"-purging already, where purging takes place in the same time as a user requesting delisting, but there should be a manual purge-request in the API too.
Deleting a host from the blacklist means that we actually only flags the host as deleted. This is being made as a part of the penalty system, but also to be able to track returning hosts.
By adding a purge-request-flag we should also be able to send a request of purging entries in the database. Sometimes, hosts are getting deprecated or added by mistake. Adding penalties to such hosts is just not fair. Instead, they should be physically removed.