This post would reflect on few essential APIs in Drupal 7 and how they were changed in Drupal 8:
drupal_get_title(): Gets the title of the current page. Link to changelog here .
$title = drupal_get_title();
$request = \Drupal::request();
$route_match = \Drupal::routeMatch();
$title = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());