Since HTTPS everywhere happened there are no truly public proxies any more, but you often still work with reverse proxies, load balancers, WAF middleboxes, and

Since HTTPS everywhere happened there are no truly public proxies any more, but you often still work with reverse proxies, load balancers, WAF middleboxes, and CDNs. A standard still helps unify things across vendors/implementations. A cacheable HTTP request is still an HTTP request, even if you use some other method to purge it later.