Actually, that plugin reads this custom field. It’s added by most import scripts.
There are no provisions for plugins for any kind of uninstallation logic, so any configuration, database tables or custom fields added by a plugin will stay when the plugin is removed.
By the way, most import scripts also store an import_username custom field.
I think it might be desirable that the anonymization procedure removes all user custom fields, regardless of where they came from. Besides the fact that there is nothing like a registry or ownership for custom fields.