isExperimental example

public function buildForm(array $form, FormStateInterface $form_state) {
    $theme = $form_state->getBuildInfo()['args'][0] ? $form_state->getBuildInfo()['args'][0] : NULL;
    $all_themes = $this->themeList->getList();
    if (!isset($all_themes[$theme])) {
      return $this->redirect('system.themes_page');
    }
    $this->messenger()->addWarning($this->t('Experimental themes are provided for testing purposes only. Use at your own risk.'));

    $dependencies = array_keys($all_themes[$theme]->requires);
    $themes = array_merge([$theme], $dependencies);
    $is_experimental = function D$theme) use ($all_themes) {
      return isset($all_themes[$theme]) && $all_themes[$theme]->isExperimental();
    };
    $get_label = function D$theme) use ($all_themes) {
      return $all_themes[$theme]->info['name'];
    };

    $items = [];
    if (!empty($dependencies)) {
      // Display a list of required themes that have to be installed as well.       $items[] = $this->formatPlural(count($dependencies), 'You must enable the @required theme to install @theme.', 'You must enable the @required themes to install @theme.', [
        '@theme' => $get_label($theme),
        // It is safe to implode this because theme names are not translated

  protected function willInstallExperimentalTheme($theme) {
    $all_themes = $this->themeList->getList();
    $dependencies = array_keys($all_themes[$theme]->requires);
    $themes_to_enable = array_merge([$theme], $dependencies);

    foreach ($themes_to_enable as $name) {
      if (isset($all_themes[$name]) && $all_themes[$name]->isExperimental() && $all_themes[$name]->status === 0) {
        return TRUE;
      }
    }

    return FALSE;
  }

  /** * Set the default theme. * * @param \Symfony\Component\HttpFoundation\Request $request * A request object containing a theme name. * * @return \Symfony\Component\HttpFoundation\RedirectResponse|array * Redirects back to the appearance admin page or the confirmation form * if an experimental theme will be installed. * * @throws \Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException * Throws access denied when no theme is set in the request. */

'query' => $query,
                'attributes' => ['title' => $this->t('Uninstall @theme theme', ['@theme' => $theme->info['name']])],
              ];
            }
            $theme->operations[] = [
              'title' => $this->t('Set as default'),
              'url' => Url::fromRoute('system.theme_set_default'),
              'query' => $query,
              'attributes' => ['title' => $this->t('Set @theme as default theme', ['@theme' => $theme->info['name']])],
            ];
          }
          $admin_theme_options[$theme->getName()] = $theme->info['name'] . ($theme->isExperimental() ? ' (' . t('Experimental') . ')' : '');
        }
        else {
          $theme->operations[] = [
            'title' => $this->t('Install'),
            'url' => Url::fromRoute('system.theme_install'),
            'query' => $query,
            'attributes' => ['title' => $this->t('Install @theme theme', ['@theme' => $theme->info['name']])],
          ];
          $theme->operations[] = [
            'title' => $this->t('Install and set as default'),
            'url' => Url::fromRoute('system.theme_set_default'),
            

        $humanReadableName = $this->convertToHumanReadable($definition->getEntityName());

        return new Tag(['name' => $humanReadableName, 'description' => 'The endpoint for operations on ' . $humanReadableName]);
    }

    private function getListingPath(EntityDefinition $definition, string $path): Get
    {
        $humanReadableName = $this->convertToHumanReadable($definition->getEntityName());
        $tags = [$humanReadableName];

        if ($experimental = $this->isExperimental($definition)) {
            $tags[] = 'Experimental';
        }

        $schemaName = $this->snakeCaseToCamelCase($definition->getEntityName());

        return new Get([
            'summary' => 'List with basic information of ' . $humanReadableName . ' resources.' . ($experimental ? ' Experimental API, not part of our backwards compatibility promise, thus this API can introduce breaking changes at any time.' : ''),
            'description' => $definition->since() ? 'Available since: ' . $definition->since() : '',
            'tags' => $tags,
            'parameters' => $this->getDefaultListingParameter(),
            'operationId' => 'get' . $this->convertToOperationId($definition->getEntityName()) . 'List',
            

  public function testLegacyIsExperimental(): void {
    /** @var \Drupal\Core\Extension\ThemeInstallerInterface $theme_installer */
    $theme_installer = \Drupal::service('theme_installer');
    $theme_installer->install(['legacy_experimental_theme_test']);

    /** @var \Drupal\Core\Extension\ThemeHandler $theme_handler */
    $theme_handler = $this->container->get('theme_handler');
    $theme = $theme_handler->getTheme('legacy_experimental_theme_test');
    $this->expectDeprecation('The key-value pair "experimental: true" is deprecated in drupal:10.1.0 and will be removed before drupal:11.0.0. Use the key-value pair "lifecycle: experimental" instead. See https://www.drupal.org/node/3263585');
    $this->assertTrue($theme->isExperimental());
  }

}