Mostly anecdotal, since there's no real definitive way to tell.

There's two spaces I see.

- Caching within the webserver, as the caching there has improved over time. The caches in Nginx, Apache, LiteSpeed, etc, perform much better than they did in the past.

- Caching at a load balancer tier. Nginx again (though in a LB context), Haproxy, etc.

Both spaces seem to have less talk about Varnish than they used to, and more about other platforms.

You can see a plateau, then decline (though slight) that starts for Varnish around mid-2018, here:

https://trends.builtwith.com/Web-Server/Varnish

Compare to:

LiteSpeed: https://trends.builtwith.com/Web-Server/LiteSpeed

Nginx: https://trends.builtwith.com/Web-Server/nginx

(Though these kinds of surveys are tricky, as they depend on outward facing headers that don't always exist, or don't always tell you enough...like Nginx doesn't always imply the cache is in use)

I also wonder what percentage hit Varnish might take if Fastly moves away. I'm sure they regret not varnish specifically, but exposing the varnish vcl directly to end users.

Edit: Yes, I agree that sites like "builtwith" are flawed, and mentioned some of the reasons above. And, I didn't mean for this comment to sound like a criticism...just an observation. I noticed builtwith's chart has a similar plateau + slight decline for Apache, starting also near mid-2018.