Source

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

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

		$data['nav_menu_updates'][] = array(
			'term_id'          => $this->term_id,
			'previous_term_id' => $this->previous_term_id,
			'error'            => $this->update_error ? $this->update_error->get_error_code() : null,
			'status'           => $this->update_status,
			'saved_value'      => 'deleted' === $this->update_status ? null : $this->value(),
		);

		$data['widget_nav_menu_updates'] = array_merge(
			$data['widget_nav_menu_updates'],
			$this->_widget_nav_menu_updates
		);
		$this->_widget_nav_menu_updates  = array();

		return $data;
	}