The updated code does check the value of #upload_validators before this property is added by the parent::formElement() causing PHP warnings:
Warning: Undefined array key "#upload_validators" in Drupal\s3fs_cors\Plugin\Field\FieldWidget\S3fsCorsFileWidget->formElement() (line 106 of modules/contrib/s3fs_cors/src/Plugin/Field/FieldWidget/S3fsCorsFileWidget.php).
Warning: Trying to access array offset on value of type null in Drupal\s3fs_cors\Plugin\Field\FieldWidget\S3fsCorsFileWidget->formElement() (line 106 of modules/contrib/s3fs_cors/src/Plugin/Field/FieldWidget/S3fsCorsFileWidget.php).
Warning: Trying to access array offset on value of type null in Drupal\s3fs_cors\Plugin\Field\FieldWidget\S3fsCorsFileWidget->formElement() (line 106 of modules/contrib/s3fs_cors/src/Plugin/Field/FieldWidget/S3fsCorsFileWidget.php).
Deprecated function: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in Drupal\Component\Utility\Bytes::toNumber() (line 76 of core/lib/Drupal/Component/Utility/Bytes.php).
Deprecated function: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in Drupal\Component\Utility\Bytes::toNumber() (line 78 of core/lib/Drupal/Component/Utility/Bytes.php).
Open the field widget
Active
1.0
Code
Should be OK now - no PHP warnings + all validators are preserved
Automatically closed - issue fixed for 2 weeks with no activity.