Condition to allow showing only on a 403/404 page.

ErrorPages.php

<?php

namespace Drupal\example\Plugin\Condition;

use Drupal\Core\Condition\ConditionPluginBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Http\RequestStack;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Condition to check for 403/403 page.
 *
 * @Condition(
 *   id = "error_pages",
 *   label = @Translation("Error pages")
 * )
 */
class ErrorPages extends ConditionPluginBase implements ContainerFactoryPluginInterface {

  /**
   * Request Stack service.
   *
   * @var \Drupal\Core\Http\RequestStack
   */
  protected RequestStack $requestStack;

  /**
   * {@inheritdoc}
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, RequestStack $request_stack) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->requestStack = $request_stack;
  }

  /**
   * @inheritDoc
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('request_stack')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['error_pages'] = [
      '#type' => 'checkboxes',
      '#title' => $this->t('Error pages'),
      '#options' => [
        '403' => $this->t('403 (access denied) page'),
        '404' => $this->t('404 (not found) page'),
      ],
      '#default_value' => $this->configuration['error_pages'],
    ];

    return parent::buildConfigurationForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['error_pages'] = array_filter($form_state->getValue('error_pages'));
    parent::submitConfigurationForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function evaluate() {
    // Returns true if no bundles are selected and negate option is disabled.
    if (empty($this->configuration['error_pages']) && !$this->isNegated()) {
      return TRUE;
    }

    $request = $this->requestStack->getCurrentRequest();
    $parameters = $request->isMethod('GET') ? $request->query : $request->request;
    if ($parameters->has('_exception_statuscode') && in_array($parameters->get('_exception_statuscode'), $this->configuration['error_pages'])) {
      return TRUE;
    }

    return FALSE;
  }

  /**
   * {@inheritdoc}
   */
  public function summary() {
    if (count($this->configuration['error_pages']) > 1) {
      $error_pages = $this->configuration['error_pages'];
      $last = array_pop($error_pages);
      $error_pages = implode(', ', $error_pages);
      return $this->t('The error page is @error_pages or @last', ['@error_pages' => $error_pages, '@last' => $last]);
    }
    $bundle = reset($this->configuration['bundles']);
    return $this->t('The error page is @error_pages', ['@error_pages' => $bundle]);
  }

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return ['error_pages' => []] + parent::defaultConfiguration();
  }

}