D8 Webforms Predefined Options
You can alter the options in this hook:

/**
 * Alter form options by id.
 *
 * @param array $options
 *   An associative array of options.
 * @param array $element
 *   The form element that the options is for.
 * @param string $options_id
 *   The form options id. Set to NULL if the options are custom.
 */
function hook_webform_options_alter(array &amp;$options, array &amp;$element, $options_id = NULL) {


See http://cgit.drupalcode.org/webform/tree/webform.api.php

The options are stored in the config entity webform_options, see http://cgit.drupalcode.org/webform/tree/src/Entity/WebformOptions.php