Drupal 8 has_page_title

node.html.twig

{#
/**
 * @file
 * Theme override to display a node.
 *
 * Available variables:
 * - see Stable Theme's node.twig.html for standard variables.
 * - has_page_title: Flag for page title block visibility. Will be true when
 *   the iu_page_title block is present on the page. This is useful to add the
 *   H1 inside the article when page title block not present.
 *
 * @see template_preprocess_node()
 * @see themename_preprocess_node()
 */
#}
<article{{ attributes }}>

  {{ title_prefix }}
  {% if page and not has_page_title %}
    <h1{{ title_attributes }}>{{ label }}</h1>
  {% elseif not page %}
    <h2{{ title_attributes }}>
      <a href="{{ url }}" rel="bookmark">{{ label }}</a>
    </h2>
  {% endif %}
  {{ title_suffix }}

  {% if display_submitted %}
    <footer>
      {{ author_picture }}
      <div{{ author_attributes }}>
        {% trans %}Submitted by {{ author_name }} on {{ date }}{% endtrans %}
        {{ metadata }}
      </div>
    </footer>
  {% endif %}

  <div{{ content_attributes }}>
    {{ content }}
  </div>

</article>

themename.theme

/**
 * Prepares variables for `node.html.twig`.
 */
function themename_preprocess_node(&$variables) {
  static $cacheable_metadata_list = [];
  $blockRepository = \Drupal::service('block.repository');
  /** @var $blocks \Drupal\block\BlockInterface[] */
  $blocks = $blockRepository->getVisibleBlocksPerRegion($cacheable_metadata_list);
  $variables['has_page_title'] = (isset($blocks['page_title']) && !empty($blocks['page_title']['themename_page_title']));
}