Upgrade Your Analytics Cookies to fight ITP2.1:

callback-example.js

/* GTM     : use the following code inside a customTask, hitCallback oder a cleanup tag  
   GA.JS   : modify your existing tracking code with a customTask or hitCallback and execute the following code  
   GTAG.JS : modify your existing tracking code with a manual page_view event 
             including an event_callback (gtag.js) and execute the following code
   
   customTask ga.js - see: https://developers.google.com/analytics/devguides/collection/analyticsjs/tasks#adding_to_a_task
   hitCallback ga.js - see: https://developers.google.com/analytics/devguides/collection/analyticsjs/sending-hits#hitcallback 
   event_callback gtag.js - see: https://developers.google.com/analytics/devguides/collection/gtagjs/sending-data#implement_event_callback_functions 
*/   

//"harden" _ga cookie by calling server-side cookie renewal script to upgrade cookie
var ua = navigator.userAgent.toLowerCase(); 
var req = null;
//check browser - upgrade only for Safari users. This condition is optional as other browsers like Firefox wll likely follow 
if ((ua.indexOf('safari') != -1) && (ua.indexOf('chrome') < 0)) { 
  if (window.XMLHttpRequest) req = new XMLHttpRequest();
  if (req != null) {
    //configured "upgrade_ga_cookie.php" script on your server required in order to work! 
    //see https://gist.github.com/mbaersch/70bb1c1ee2ea4787544d1ed395de1727 for details
    req.open("GET", "/upgrade_ga_cookie.php", true);
    req.send();
  }
}

gtag-example.html

<!-- complete example for gtag.js tracking code with cookie renewal event-callback to upgrade_ga_cookie.php 
(see example below) and deactvated cookie_update through ga´s tracking code. Sending the automatic pageview 
is deactvated and a separate event ist used to add the callback function. Uses own cookie to renew _ga cookie 
server-side every 6 days - see Simo Ahava´s example at 
https://disqus.com/home/discussion/simoahava/create_a_cookie_rewrite_web_service_using_the_google_cloud_platform/

usage: replace (both!) UA-1234567890-1 with your own property id -->

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-1234567890-1"></script>
<script>
  function getCookie(cname) {
    var name = cname + "=";
    var decodedCookie = decodeURIComponent(document.cookie);
    var ca = decodedCookie.split(';');
    for(var i = 0; i <ca.length; i++) {
      var c = ca[i];
      while (c.charAt(0) == ' ') {
        c = c.substring(1);
      }
      if (c.indexOf(name) == 0) {
        return c.substring(name.length, c.length);
      }
    }
    return "";
  }

  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', 'UA-1234567890-1', {'cookie_update': false, 'anonymize_ip': true, 'send_page_view': false});
  gtag('event', 'page_view', {
    'event_callback': function() {
      var cid = getCookie('_ga');
      var req = null;
      var didUpdate = (getCookie('ga_updated') === "true");
      if (cid && !didUpdate) {
        if (window.XMLHttpRequest) req = new XMLHttpRequest();
        if (req != null) {
          req.open("GET", "/upgrade_ga_cookie.php", true);
          req.send();
          document.cookie = 'ga_updated=true;path=/;max-age=' + 60*60*24*6;
        }
      }
    }
  });
  
</script>

universal-example.html

<!-- complete example for analytics.js tracking code with cookie renewal hitCallback to upgrade_ga_cookie.php 
(see example below) and deactivated cookie_update through ga´s tracking code. Uses own cookie to renew _ga cookie 
server-side every 6 days - see Simo Ahava´s example at 
https://disqus.com/home/discussion/simoahava/create_a_cookie_rewrite_web_service_using_the_google_cloud_platform/

usage: replace UA-1234567890-1 with your own property id -->

<script>

  function getCookie(cname) {
    var name = cname + "=";
    var decodedCookie = decodeURIComponent(document.cookie);
    var ca = decodedCookie.split(';');
    for(var i = 0; i <ca.length; i++) {
      var c = ca[i];
      while (c.charAt(0) == ' ') {
        c = c.substring(1);
      }
      if (c.indexOf(name) == 0) {
        return c.substring(name.length, c.length);
      }
    }
    return "";
  }

  (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
  (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
  })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
  ga('create', 'UA-1234567890-1', 'auto', {cookieUpdate: false});
  ga('set', 'anonymizeIp', true);
  ga('send', 'pageview', {
    hitCallback: function() {
      var cid = getCookie('_ga');
      var req = null;
      var didUpdate = (getCookie('ga_updated') === "true");
      if (cid && !didUpdate) {
        if (window.XMLHttpRequest) req = new XMLHttpRequest();
        if (req != null) {
          req.open("GET", "/upgrade_ga_cookie.php", true);
          req.send();
          document.cookie = 'ga_updated=true;path=/;max-age=' + 60*60*24*6;
        }
      } 
    }
  });
    
</script> 

upgrade_ga_cookie.php

<?php
  //SETUP: set this according to the cookie domain of your Google Analytics implementation 
  $domain = ".mydomain.de";

  //get existing _ga cookie value
  $cid = ($_COOKIE['_ga']); 
  
  //renew cookie with 2 years lifetime
  if ($cid) 
    setcookie("_ga", $cid, time()+60*60*24*365*2, "/", $domain);
?>

As described in Simo Ahava´s blogpost about a rewrite web service the number of "renewal calls" can be reduced by stopping GA from updating the cookie every time. Remembering the last update in a separate cookie and only sending the request if neccessary can be achieved in the same way. The only difference would be that no web service gets called but the upgrade_ga_cookie.php on your own server.

I added two examples for gtag.js and analytics.js