// pass youtube to youtube-nocookie
add_filter('render_block',
  function ($block_content, $block) {
  // filter block
  if ($block['blockName'] === 'core/embed') {
    $tags = new \WP_HTML_Tag_Processor($block_content);
    $tags->next_tag('iframe');
    $url = $tags->get_attribute('src');
    $parameters = parse_url($url);

    // check if youtube
    if ($parameters['host'] !== 'www.youtube.com') {
      return $block_content;
    }

    // update host
    $parameters['host'] = 'www.youtube-nocookie.com';
    // add rel=0 for related video. Show only the video from the
    // current channel, remove the next line if you want to show
    // related videos from other channels
    $parameters['query'] = $parameters['query'] === '' ? 'rel=0' : 'rel=0&'.$parameters['query'];
    // reconstruct url
    $url = $parameters['scheme'].'://'.$parameters['host'].$parameters['path'].'?'.$parameters['query'];
    $tags->set_attribute('src', $url);

    return $tags;
}

  return $block_content;

}, 1, 2);