In case anyone comes across this looking for the correct way as of Drupal 8.8.0 the sync directory is declared like this in settings.php:

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';

See: https://www.drupal.org/node/3018145

In terms of location, it's generally considered to be more secure if it's below the webroot, e.g.:

$settings['config_sync_directory'] = '../config/sync';

Further reading: https://www.drupal.org/docs/configuration-management/changing-the-storage-location-of-the-sync-directory

core.extension.yml is a YAML file listing all installed and enabled modules, themes, and profiles and their respective weight (if specified).

The correct way of declaring the sync directory in settings.php changed with Drupal 8.8.0 as follows:

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';

See: https://www.drupal.org/node/3018145

In terms of location, it's generally considered to be more secure if it's below the webroot, e.g.:

$settings['config_sync_directory'] = '../config/sync';

Further reading: https://www.drupal.org/docs/configuration-management/changing-the-storage-location-of-the-sync-directory

In case anyone comes across this looking for the correct way as of Drupal 8.8.0 the sync directory is declared like this in settings.php:

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';

See: https://www.drupal.org/node/3018145

In case anyone comes across this looking for the correct way as of Drupal 8.8.0 the sync directory is declared like this in settings.php:

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';

See: https://www.drupal.org/node/3018145

In terms of location, it's generally considered to be more secure if it's below the webroot, e.g.:

$settings['config_sync_directory'] = '../config/sync';

Further reading: https://www.drupal.org/docs/configuration-management/changing-the-storage-location-of-the-sync-directory