code: def get_all_allowed_plugins(self):
          
          # Get all allowed plugins of the various placeholders together.
          if not hasattr(self.model, '_meta_placeholder_fields'):
              # No placeholder fields in the model, no need for inlines.
              return []
  
          plugins = []
          for name, field in self.model._meta_placeholder_fields.items():
              assert isinstance(field, PlaceholderField)
              if field.plugins is None:
                  # no limitations, so all is allowed
                  return extensions.plugin_pool.get_plugins()
              else:
                  plugins += field.plugins
  
          return list(set(plugins))
docs: Return which plugins are allowed by the placeholder fields.