Tried that filter, but still json was created on my clients site.
Edit: With above filter for acf/settings/json the data from json is not included in the post edit screen as expected.
But with WPML plugin active, the json folder and files are still created because the settings_save_json filter in core/wpml.php always creates the json folder if it does not yet exist and due to this is_writable() in update_field_group in core/json.php will always return true.
As a solution I would suggest to add the acf_get_setting('json') validation from include_fields also to update_field_group and delete_field_group in core/json.php.
WP 3.8.3, ACF PRO 5.0.0, WPML 3.1.6
