When running cleanup_database.pl with the --del-exp-selfreg flag to delete selfref patrons, Koha will delete a patron who is a guarantor. It should prevent this, as it does when batch deleting patrons from the staff interface.
Making some squeaky noises on this! It would be nice to know that guarantors won't be deleted without staff intervention. Some other granular controls for selfreg clean-up (so we don't accidentally delete test patrons) would also be nice. -Marti Fuerst