The sync directory is defined in $settings and not $config_directories

The configuration sync directory is now defined in $settings['config_sync_directory'] in settings.php.

The ability to support multiple configuration directories in the $config_directories is deprecated. If you have custom or contributed code that relies on this ability then you need to move your setting either to $settings or another storage - for example state or configuration itself.

As a result of simplifying configuration sync directory configuration we have also deprecated config_get_config_directory(), CONFIG_SYNC_DIRECTORY and drupal_install_config_directories().

Instead of calling config_get_config_directory(CONFIG_SYNC_DIRECTORY) use Settings::get('config_sync_directory').

The constant CONFIG_SYNC_DIRECTORY has no replacement because it is not necessary anymore. The install method drupal_install_config_directories() also has no replacement. It's functionality is implemented as part of \Drupal\Core\Installer\Form\SiteSettingsForm::submitForm().

Updating your settings.php file

Before

$config_directories['sync'] = 'sites/default/files/config_YLZJmmpOqc_KBWbMc2I58ky3-8c7qtg4G-OpSqFClHs5E0NL9YMFgyF4RRTv8IFdl_kAMs_Bdw/sync';

After

$settings['config_sync_directory'] = 'sites/default/files/config_YLZJmmpOqc_KBWbMc2I58ky3-8c7qtg4G-OpSqFClHs5E0NL9YMFgyF4RRTv8IFdl_kAMs_Bdw/sync';