Drupal8 still makes massive use of "by convention" structured arrays

namespace Drupal\page_example\Controller;
 
use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
 
/**
 * Controller routines for page example routes.
 */
class PageExampleController {
 
  /**
   * Constructs a page with descriptive content.
   *
   * Our router maps this method to the path 'examples/page_example'.
   */
  public function description() {
    $build = array(
      '#markup' => t('<p>The Page example module provides two pages, "simple" and "arguments".</p><p>The <a href="@simple_link">simple page</a> just returns a renderable array for display.</p><p>The <a href="@arguments_link">arguments page</a> takes two arguments and displays them, as in @arguments_link</p>',
        array(
          '@simple_link' => url('examples/page_example/simple', array('absolute' => TRUE)),
          '@arguments_link' => url('examples/page_example/arguments/23/56', array('absolute' => TRUE)),
        )
      ),
    );
 
    return $build;
  }
 
  /**
   * Constructs a simple page.
   *
   * The router _content callback, maps the path 'examples/page_example/simple'
   * to this method.
   *
   * _content callbacks return a renderable array for the content area of the
   * page. The theme system will later render and surround the content with the
   * appropriate blocks, navigation, and styling.
   */
  public function simple() {
    return array(
      '#markup' => '<p>' . t('Simple page: The quick brown fox jumps over the lazy dog.') . '</p>',
    );
  }
 
  /**
   * A more complex _content callback that takes arguments.
   *
   * This callback is mapped to the path
   * 'examples/page_example/arguments/{first}/{second}'.
   *
   * The arguments in brackets are passed to this callback from the page URL.
   * The placeholder names "first" and "second" can have any value but should
   * match the callback method variable names; i.e. $first and $second.
   *
   * This function also demonstrates a more complex render array in the returned
   * values. Instead of rendering the HTML with theme('item_list'), content is
   * left un-rendered, and the theme function name is set using #theme. This
   * content will now be rendered as late as possible, giving more parts of the
   * system a chance to change it if necessary.
   *
   * Consult @link http://drupal.org/node/930760 Render Arrays documentation
   * @endlink for details.
   *
   * @param string $first
   *   A string to use, should be a number.
   * @param string $second
   *   Another string to use, should be a number.
   *
   * @throws \Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException
   *   If the parameters are invalid.
   */
  public function arguments($first, $second) {
    // Make sure you don't trust the URL to be safe! Always check for exploits.
    if (!is_numeric($first) || !is_numeric($second)) {
      // We will just show a standard "access denied" page in this case.
      throw new AccessDeniedHttpException();
    }
 
    $list[] = t("First number was @number.", array('@number' => $first));
    $list[] = t("Second number was @number.", array('@number' => $second));
    $list[] = t('The total was @number.', array('@number' => $first + $second));
 
    $render_array['page_example_arguments'] = array(
      // The theme function to apply to the #items
      '#theme' => 'item_list',
      // The list itself.
      '#items' => $list,
      '#title' => t('Argument Information'),
    );
    return $render_array;
  }
}