I am more in favor of leaving default_topics_automatic_unpin untouched so it matches all the other default_xxx settings that configure the defaults for user preferences. That follows a consistent naming pattern like default_hide_profile or default_other_enable_smart_lists. And for me, it also makes sense to group all these in the admin > user defaults config section.
Maybe automatically_unpin_topics could just be hidden? Most preferences don’t have another setting to prevent users from changing them. disable_mailing_list_mode is the only one that comes to my mind. I understand why that’s something admins want to disable entirely, but I don’t see this reason for automatic unpin. If a user really wants to get rid of pins, they will also unpin topics manually. It feels like the user preference with the default setting should have just replaced the global setting.