If any of these deprecated features is a cause for concern for you, please email the curl-library mailing list as soon as possible and explain to us why this is a problem for you and how your use case can't be satisfied properly using a work around.
HTTP pipelining is badly supported by curl in the sense that we have bugs and it is a fragile feature without enough tests. Also, when something turns out to have problems it is really tricky to debug due to the timing sensitivity so very often enabling debug outputs or similar completely changes the nature of the behavior and things are not reproducing anymore!
HTTP pipelining was never enabled by default by the large desktop browsers due to all the issues with it. Both Firefox and Chrome have also dropped pipelining support entirely since a long time back now. We are in fact over time becoming more and more lonely in supporting pipelining.
The bad state of HTTP pipelining was a primary driving factor behind HTTP/2 and its multiplexing feature. HTTP/2 multiplexing is truly and really “pipelining done right”. It is way more solid, practical and solves the use case in a better way with better performance and fewer downsides and problems.
In 2018, pipelining should be abandoned and HTTP/2 should be used instead.
In 7.62.0, we will add code that ignores the “enable pipeline” option setting). The *setopt() function would still return “OK” though so the application couldn't tell that this is happening.
Users who truly need pipelining from that version will need to modify the code (ever so slightly) and rebuild.
Six months later, in sync with the planned release happen in April 2019, (might be 7.66.0), assuming no major riots have occurred due to this in the mean time, we rip out the pipelining code. It is in the order of 1000 lines of libcurl code.
Left to answer: should the *setopt() function start to return error when these options are set to be able to tell when they're trying to use options that are no longer around or should we maintain behavior as much as possible?
CURLOPT_DNS_USE_GLOBAL_CACHEThis option makes libcurl use a global non-thread-safe cache for DNS if enabled. The option has been marked as “obsolete” in the header file and in documentation for several years already.
There's proper and safe method alternative provided since many years: the share API.
In curl 7.62.0 setting this option to TRUE will not have any effect. The global cache will not be enabled. The code still remains so it is easy to revert if need be.
Remove all global-cache related code from curl around April 2019 (might be 7.66.0).