I was under the impression that when AnonymousPatron is filled with a borrowernumber, deleting a patron (individually or with batch deletion tool) would anonymize their circulation history using AnonymousPatron, but it doesn't. After deleting a or some patrons, the circulation history in the book record shows blank spaces in lieu of the patrons. It would make more sens that when deleting a patron using the batch deletion tool or the delete button in a patron's page, the AnonymousPatron is used to fill the circulation history of the deleted patrons.
It will be used when a patron or library decides to anonymize the history (patron privacy settings). Why do you think anonymous would make more sense than null? Or are the whole lines/entries gone?
No, the entries are not gone, but to me it would make more sens to group all of the old issues that don't have a patron anymore (because of anonymization or deletion) under the AnonymousPatron's name so that if they look at their stats or in the circulation history of a document, they don't have null entries AND entries under the AnonymousPatron's name. If they use an SQL query to look at their old issues and they group the issues using different borrower's criterias, they would have a null entry and then an entry for the AnonymousPatron. It would just be more user-friendly I think.