Template suggestions based on path alias
OK, I seem to have got it working.

function THEME_NAME_theme_suggestions_page_alter(array &amp;$suggestions, array $variables) {
  $current_path = \Drupal::service('path.current')-&gt;getPath();
  $result = \Drupal::service('path.alias_manager')-&gt;getAliasByPath($current_path);

  $path_alias = trim($result, '/');
  $path_alias = str_replace('/', '-', $path_alias);

  $suggestions[] = 'page__path_alias__'.$path_alias;
}