Guest Meta for WordPress - allows you to store meta-data for guests just like you would for users.

guest-meta.php

<?php
/*
Plugin Name: Guest Meta
Plugin URI: <https://gist.github.com/lordmatt/3bd8f7787fbbe0950f9228dec92f2f8a>
Description: Enable storing meta data for guests. Keeps cookies small and simple.
Author: Matthew Brown
Author URI: http://matthewdbrown.authorbuzz.co.uk/
Version: 1.1.0
License: GPLv3 or later
Text Domain: guest_meta
*/

/**
 * This is an idea I had to store meta data about guest users as well as regular
 * registered users. In theory, installing this as a plugin will give your other
 * plugins and themes the ability to store guest metadata against a cookie value
 * and not in the cookie which means the data is tamper proof and more trustable
 * in the main.
 * 
 * This untested draft assumes everything I read in the WordPress' documentation 
 * is 100% accurate.
 * 
 * @package     guest_meta
 * @author      Matthew Brown <https://twitter.com/lordmatt>
 * @copyright   (C) 2019 Matthew Brown.
 * @license     http://www.gnu.org/licenses/old-licenses/gpl-2.0-standalone.html 
 * @used-by		storyteller.datastore <https://gist.github.com/lordmatt/e6323ae00a39373841344ebb0b49b8fd> 
 * 
 * VERSION: 1.1.0
 * 
 * =======
 * History
 * =======
 * 1.1.0 - Added pass FALSE or 0 as guest-ID and the ID will autoload
 * 1.0.0 - First version. Not tested, yet.
 * 
 * =======
 * License
 * =======
 * 
 * This file is part of guest_meta.
 * 
 * guest_meta is free software: you can redistribute it and/or modify it under 
 * the terms of the GNU General Public License as published by the Free Software 
 * Foundation, either version 2 of the License, or (at your option) any later 
 * version.
 * 
 * guest_meta is distributed in the hope that it will be useful, but WITHOUT ANY 
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR 
 * A PARTICULAR PURPOSE. See the GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License along with 
 * guest_meta. If not, see http://www.gnu.org/licenses/.
 */


function guest_meta_install(){
	global $wpdb;
	$table_name = $wpdb->prefix . "guestmeta"; 
	$charset_collate = $wpdb->get_charset_collate();

	$sql = "CREATE TABLE $table_name (
	  `meta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
	  `object_id` bigint(20) unsigned NOT NULL DEFAULT '0',
	  `meta_key` varchar(255) DEFAULT NULL,
	  `meta_value` longtext,
	  `ts` TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
	  PRIMARY KEY  (meta_id)
	) $charset_collate;";

	$sql2 = "ALTER TABLE $table_name
	  ADD KEY `comment_id` (`object_id`),
	  ADD KEY `meta_key` (`meta_key`(191));";

	require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
	dbDelta( $sql );
	dbDelta( $sql2 );
}

register_activation_hook( __FILE__, 'guest_meta_install' );
add_action( 'init', 'guest_meta_cookie' );

function guest_meta_cookie() {
	setcookie( 'guest_meta', guest_meta_get_object_id(), 30 * DAY_IN_SECONDS, COOKIEPATH, COOKIE_DOMAIN );
}

function guest_meta_get_object_id(){
	if(!isset($_COOKIE['guest_meta'])) {
		$_COOKIE['guest_meta'] = guest_meta_make_id();
	}
	return $_COOKIE['guest_meta'];
}

function guest_meta_make_id(){
	global $wpdb;
	$table_name = $wpdb->prefix . "guestmeta";
	guest_meta_drop_old_rows(); // clean up old stuff first
	$results = $wpdb->get_results( "SELECT MAX(object_id) AS almost FROM {$table_name};", OBJECT );
	$newish = $results->almost+1;
	return $newish;
}

function guest_meta_drop_old_rows(){
	global $wpdb;
	$table_name = $wpdb->prefix . "guestmeta";
	$sql = "DELETE FROM $table_name WHERE `ts` < UNIX_TIMESTAMP(DATE_SUB(NOW(), INTERVAL 28 DAY))";
	require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
	dbDelta( $sql );
}

function add_guest_meta($object_id=FALSE, $meta_key=FALSE, $meta_value=FALSE, $unique=FALSE){
	if($object_id==FALSE||$object_id<1){
		$object_id = guest_meta_get_object_id();
	}
	return add_metadata( 'guest', $object_id, $meta_key, $meta_value, $unique );
}

function get_guest_meta($object_id=FALSE, $meta_key=FALSE, $single=FALSE){
	if($object_id==FALSE||$object_id<1){
		$object_id = guest_meta_get_object_id();
	}
	get_metadata('guest', $object_id, $meta_key, $single);
}

function update_guest_meta($object_id=FALSE, $meta_key=FALSE, $meta_value=FALSE, $prev_value=''){
	if($object_id==FALSE||$object_id<1){
		$object_id = guest_meta_get_object_id();
	}
	update_metadata( 'guest', $object_id, $meta_key, $meta_value, $prev_value );
}

function delete_guest_meta($object_id=FALSE, $meta_key=FALSE, $meta_value=FALSE, $delete_all=FALSE){
	if($object_id==FALSE||$object_id<1){
		$object_id = guest_meta_get_object_id();
	}
	delete_metadata ( 'guest', $object_id, $meta_key, $meta_value, $delete_all );
}

Written to solve a problem here.