Only Composer's classloader is used

In Drupal 8 the default behaviour was to add an additional classloader to enable caching in APCu. This has been removed in Drupal 9 in favour of using Composer's classloader. The default behaviour is still to use APCu to cache class locations.

Previously, to use composer's APCu optimised classloader you would run composer install with the --apcu-autoloader and add the $settings['class_loader_auto_detect'] = FALSE; to your settings.php. This is no longer necessary and can be removed.

See this change record if you were using WinCache.