I encountered this when I had to build a hero paragraph. I needed the page title inside my paragraph.
This is how I did it. To my mytheme.theme file I added
function mytheme_preprocess_paragraph(&$variables) {
$paragraph = $variables['paragraph'];
// if type hero
if ($paragraph->bundle() == 'hero') {
$request = \Drupal::request();
$route_match = \Drupal::routeMatch();
$variables['page_title'] = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());
$variables['#cache']['contexts'][] = 'url';
}
}
Now I can just use
{{ page_title }}