fetcher.php

// Adapted from https://www.drupal.org/project/drupal/issues/3041885
public function getSecurityAdvisoryMessages(): array {
    $messages = [];

    $response = $this->tempStore->get('advisories_response');
    if (!$response) {
      $advisories_endpoint = 'https://drupal.org/items.json';
      $response = (string) $this->httpClient->get($advisories_endpoint)->getBody();
// advisories.check_frequency is 12 hours in seconds
      $this->tempStore->setWithExpire('advisories_response', $response, $this->config->get('advisories.check_frequency'));
    }

    $json_payload = json_decode($response, TRUE);
    if ($json_payload !== NULL) {
      foreach ($json_payload as $json) {
        $output[] = [
          '#theme' => 'drupalorg_item',
          '#item' => $json,
        ];
      }
    }
    else {
      throw new \UnexpectedValueException('Drupal PSA JSON is malformed.');
    }

    return $messages;
  }