Cloudflare's proxy violates HTTP/1.1, and they don't care

Nothing to really see in these forum posts aside from at best what I can gather is, I'm right, your wrong and you should be thanking me for pointing this out.

Poster made their valid point about vary headers & Cloudflare responded stating it isn't a bug and it is their intended behavior.

Forum poster continues along the lines of, "I want to use this service but this service needs to conform to the specification and nothing less."

They're not matching the specification, they're aware and they admit it. Move providers if you require this or run your own.

I don’t recall where I read this but as I understand it Cloudflare ignores Vary: User-Agent because (when exposed to the general public) it’s effectively the same as “never cache this page,” due to the wide variety of user agents and the amount of detail they pack into their user-agent string.

It’s basically a very big hammer that (a) would massively increase the load on both Cloudflare’s cache and the backing server, and (b) almost never does what the server owner actually wants (which is usually “send a different variant on this page to IE users”).

Vary is a basic header. That is a little surprising.

Very understandable for a CDN. You can use page rules to disable caching on various levels (even in the free tier, just with limitations).