protected function checkDeprecatedKeys() {
    $deprecated_keys = [
      'project.hash_salt',
      'project.profile.contrib',
      'project.vendor',
      'project.description',
      'project.themes',
      'hosting',
    ];
    $deprecated_keys_exist = FALSE;
    $outcome = [];
    foreach ($deprecated_keys as $deprecated_key) {
      if ($this->getConfigValue($deprecated_key)) {
        $outcome[] = "The '$deprecated_key' key is deprecated. Please remove it from blt.yml.";
        $deprecated_keys_exist = TRUE;
      }
    }

    if ($deprecated_keys_exist) {
      $this->logProblem(__FUNCTION__ . ':keys', $outcome, 'comment');
    }
  }