third_party_settings_example.module

<?php

/**
 * @file
 * The third party settings example module file.
 */

/**
 * Implements hook_form_FORM_ID_alter().
 */
function third_party_settings_example_form_node_type_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  $entity = $form_state->getFormObject()->getEntity();
  if ($entity && $entity->id() === 'article') {
    $exampleTextField = $entity->getThirdPartySetting('third_party_settings_example', 'text_field');

    $form['example_settings'] = [
      '#type' => 'fieldset',
      '#tree' => FALSE,
      '#title' => t('Third Party Example Settings'),
      '#description' => t('For example purposes.'),
    ];

    $form['example_settings']['text_field'] = [
      '#type' => 'textfield',
      '#title' => t('Example Text Field'),
      '#default_value' => $exampleTextField,
    ];

    $form['#entity_builders'][] = 'third_party_settings_example_node_form_builder';
  }
}

/**
 * Entity builder for the node:article entity.
 */
function third_party_settings_example_node_form_builder($entity_type, $entity, &$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  if ($form_state->getValue('text_field')) {
    $textFieldValue = $form_state->getValue('text_field');
    $entity->setThirdPartySetting('third_party_settings_example', 'text_field', $textFieldValue);
    return;
  }
  $entity->unsetThirdPartySetting('third_party_settings_example', 'text_field');
}

/**
 * Implements hook_preprocess_HOOK().
 */
function third_party_settings_example_preprocess_node(&$variables) {
  /** @var \Drupal\node\NodeInterface $node */
  $node = $variables['elements']['#node'];
  if ($node->bundle() == 'article') {
    $additionalSetting = $node->type->entity->getThirdPartySetting('third_party_settings_example', 'text_field');

    $variables['content']['additional_title'] = [
      '#markup' => $additionalSetting,
    ];
  }
}