https://learn.akamai.com/en-us/webhelp/adaptive-media-delive...
> AMD automatically strips these headers out of requests to support caching for faster delivery.
> I need the Vary HTTP headers: AMD can cache the associated object if the Vary HTTP header contains only "Accept-Encoding" and "Gzip" is present in the Content-Encoding header
(AMD in this case standing for Akamai Media Delivery)
It wasn't that simple — IIRC, for a while Vary meant “don't cache anything, ever, under any circumstances” unless you made some custom configuration changes. Over time they _added_ support for just “Vary: Accept-Encoding” (IIRC less than a decade ago) and that was fragile. They improved that over time but it was painful for a number of years because there were various failure modes which meant things wouldn't be cached, or (IIRC) compression would be disabled for certain URLs sporadically if the first request for the option did not request transfer compression.
Content-Encoding should be well supported, User-Agent less so and for very good reasons (there's too much variation in UA strings)