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 cannot be satisfied properly using a workaround.
We remove support for building curl with the NSS TLS library in August 2022.
Starting in 7.82.0, building curl to use NSS configure requires the additional flag --with-nss-deprecated in an attempt to highlight these plans.
We make selecting NPN a no-op starting in August 2022.
Next Protocol Negotiation is a TLS extension that was created and used for agreeing to use the SPDY protocol (the precursor to HTTP/2) for HTTPS. In the early days of HTTP/2, before the spec was finalized and shipped, the protocol could be enabled using this extension with some servers.
curl supports the NPN extension with some TLS backends since then, with a command line option --npn and in libcurl with CURLOPT_SSL_ENABLE_NPN.
HTTP/2 proper is made to use the ALPN (Application-Layer Protocol Negotiation) extension and the NPN extension has no purposes anymore. The HTTP/2 spec was published in May 2015.
Today, use of NPN in the wild should be extremely rare and most likely totally extinct. Chrome removed NPN support in Chrome 51, shipped in June 2016. Removed in Firefox 53, April 2017.