[acdha]

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.