Fix remove all blocks in the content region. Resulted in losing default integration with other modules

Having the remove all blocks will lose default integration with other modules like Social Auth.
It's only allowing [Main page content]

  // Remove all blocks from the content region (such as page title and local
  // tasks) except the "Main page content" block.
  foreach (Element::children($variables['page']['content']) as $element) {
    if (isset($variables['page']['content'][$element]['#plugin_id']) && $variables['page']['content'][$element]['#plugin_id'] == 'system_main_block') {
      $variables['page']['content'] = $variables['page']['content'][$element];
      break;
    }
  }

Some sites may want to add the Title of the page or a custom title, or more options.
And custom info or functional blocks before or after the default login form