HTTP Caching, a Refresher

As is traditional with most explanations of HTTP caching, it doesn't mention Vary header. Although apparently some CDNs (e.g. Cloudflare) straight up ignore it for some reason [0].

[0] https://news.ycombinator.com/item?id=38346382

As many have pointed out here, the nature of caching has changed in the current climate of ubiquitous HTTPS, and I want to add a paragraph or two about it. Is there a good summary somewhere that I could reference? What are the the usual, most prevalent uses of HTTP intermediaries involving caches, besides CDNs and origin-controlled caches (eg Varnish)?

This is nothing new and doesn't add anything new to the topic, so am I the only that thinks this is just an attempt at boosting their SEO through HN?

A lot of this seems irrelevant these days with https everywhere.