code: private function handlePrependLocale(array $extensionConfigs, ContainerBuilder $container): array
      {
          if (!$container->hasParameter('prepend_locale')) {
              return $extensionConfigs;
          }
  
          foreach ($extensionConfigs as $extensionConfig) {
              if (isset($extensionConfig['prepend_locale'])) {
                  return $extensionConfigs;
              }
          }
  
          @trigger_error('Defining the "prepend_locale" parameter in the parameters.yml file has been deprecated and will no longer work in Contao 5.0. Define the "contao.prepend_locale" parameter in the config.yml file instead.', E_USER_DEPRECATED);
  
          $extensionConfigs[] = [
              'prepend_locale' => '%prepend_locale%',
          ];
  
          return $extensionConfigs;
      }
docs: Adds backwards compatibility for the %prepend_locale% parameter.
  
  @return array<string,array<string,mixed>>