Nowadays I almost have zero intersection with Varnish - my own impression it was much more popular like 10 years ago or even more.
I know couple of frameworks/systems support it, especially in php world.
Looks like that it's lost in layers - dev guys don't care much, sysadmins are sort of extincted, noone to bother to add Varnish into request processing queue. Needless to say, people ok HN even complain on Nginx configs,while for base caching it's much simpler, from my perspective.
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