Can't you whitelist the specific jQuery library on Google's CDN though?

Serving widely used libraries out of a CDN is a best practice for a reason. Most visitors will already have it in cache. What alternative are you supposing? Local hosting? That has drawbacks, including more cache misses and increased bandwidth costs for the website provider.

It's not always the user doing the blocking. A lot of sites broke for me when I was in China because they were trying to fetch resources from Google's CDN, which is/was blocked there.

Surely the primary blame there lies in China's censorship of the Internet, no? Isn't it just assumed that if you're traveling to China you need to be using a VPN in order to have the web as you're used to it work normally?

No doubt Google considers it a best practice, because they are in the business of knowing what people do on the internet. How often will a browser get the jquery for a particular website or check with the CDN? There are a lot of jquery versions in use, the cache gets deleted (often on browser close) or expires, cache-age=0, etc. Almost always?

Besides providing jquery themselves, websites can use a CDN from another provider. OK, that might cost an extra 20 ms.

Right, but then Google can leverage their CDN to track you, whether or not you also let them show you ads through normal channels. Ideally people would use a CDN not built to track people around.

Presumably once Google traps you into an unblockable ad channel (on mobile or whatever) they will unleash the tracking they've been doing on you.

It's really not as beneficial as it sounds in theory. Sites uses different versions of these libraries. You would be better serving a single minified asset that is cached for your users.

Websites using cdns should always use a server hosted backup, it's not that hard

  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
  <script>
  if (typeof jQuery == 'undefined')
  {
     document.write(unescape("%3Cscript src='/serverHostedJquery.js' %3E%3C/script%3E"));
  }
  </script>

Is not it better to serve scripts from your server? It gives you more control and you don't have to share your visitors data with Google. With external hosting you get more downtime, and in some countries Google servers are blocked so you get less visitors.

It's annoying to test though. You'd be better self hosting everything and putting a CDN in front of your site.