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Cache control headers not set #7008
Since version 3 uses its own session handling, session_start(), quite rightly, is not longer called. However, as session_start() was also responsible for setting the cache control headers, they are now no longer being set by default (apart from a few places such as captcha/basic and downloads).
The session.cache-limiter, which usually defaults to nocache, controls the headers. http://php.net/manual/en/session.configuration.php#ini.session.cache-limiter You can see the headers that used to be set by session_start() here. http://php.net/manual/en/function.session-cache-limiter.php
This is particularly problematic for Internet Explorer 11 (note: not Edge in emulation mode) which aggressively caches responses from ajax GET request. This can cause problems with adding to cart, checkout, payment selection, etc.
Some forum posts confirming the problem.