Source

File: wp-includes/customize/class-wp-customize-nav-menu-item-setting.php

public function amend_customize_save_response( $data ) {
		if ( ! isset( $data['nav_menu_item_updates'] ) ) {
			$data['nav_menu_item_updates'] = array();
		}

		$data['nav_menu_item_updates'][] = array(
			'post_id'          => $this->post_id,
			'previous_post_id' => $this->previous_post_id,
			'error'            => $this->update_error ? $this->update_error->get_error_code() : null,
			'status'           => $this->update_status,
		);
		return $data;
	}