Call to undefined method BaseFieldDefinition::getThirdPartySettings()

@rymcveigh, I have the same problem in a website we have built. I there use the comments as a property in a custom entity though and not using the field UI. In order to get more information, could you tell us if you are using the field UI in your entity with the comments or not?

Confirmed using a comment base field definition. 3.x depends on third_party_settings to allow distinct configurations per comment field instance. Base fields don't implement the ThirdPartySettingsInterface hence the error. However, base fields can define the third_party_settings key in the definition settings (see example below). The module code has been adjusted to support both configuration and custom base fields.

$fields['comment_extra'] = BaseFieldDefinition::create('comment')
      ->setLabel(t('Custom Comment Field'))
      ->setSetting('comment_type', 'comment')
      ->setSetting('third_party_settings', [
        'comment_delete' => [
          'operation' => [
            'hard' => 'hard',
            'hard_partial' => 'hard_partial',
            'soft' => 'soft',
          ],
          'visibility' => 'visible_multiple',
        ],
      ])
      ->setDefaultValue([
        'status' => CommentItemInterface::OPEN,
      ])
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE);

Automatically closed - issue fixed for 2 weeks with no activity.

I ran into this same issue and had to dig for a while to find this solution. Could we update the documentation in either the readme or the module description so that folks have an easier time defining the third-party settings on custom baseFieldDefinitions?