  
      @cached_property
      def related_manager_cls(self):
          return create_reverse_many_to_one_manager(
              self.rel.related_model._default_manager.__class__,
              self.rel,
          )
  
      def __get__(self, instance, cls=None):
          """
 
</s> remove             model._default_manager.__class__,
 </s> add             related_model._default_manager.__class__, </s> remove         model = self.rel.related_model if self.reverse else self.rel.model
 </s> add         related_model = self.rel.related_model if self.reverse else self.rel.model
 </s> remove     'required_db_vendor',
 </s> add     'required_db_vendor', 'base_manager_name', 'default_manager_name',
    'manager_inheritance_from_future',
            related_model._default_manager.__class__,
  
      @cached_property
      def related_manager_cls(self):
          return create_reverse_many_to_one_manager(
            related_model._default_manager.__class__,
              self.rel,
          )
  
      def __get__(self, instance, cls=None):
          """
keep keep keep keep replace keep keep keep keep keep
 Fixed #10506, #13793, #14891, #25201 -- Introduced new APIs to specify models' default and base managers.

This deprecates use_for_related_fields.

Old API:

class CustomManager(models.Model):
    use_for_related_fields = True

class Model(models.Model):
    custom_manager = CustomManager()

New API:

class Model(models.Model):
    custom_manager = CustomManager()

    class Meta:
        base_manager_name = 'custom_manager'

Refs #20932, #25897.

Thanks Carl Meyer for the guidance throughout this work.
Thanks Tim Graham for writing the docs. 
django/db/models/fields/related_descriptors.py