vquic: drop support for OpenSSL-QUIC

- It is slower and uses more memory than the alternatives and is only
  experimental in curl.
- We disable a few tests for OpenSSL-QUIC because of flakiness
- It gets little attention from OpenSSL and we have no expectation of the
  major flaws getting corrected anytime soon.
- No one has spoken up for keeping it
- curl users building with vanilla OpenSSL can still use QUIC through the
  means of ngtcp2

Closes #20226

docs/DEPRECATE.md

@@ -16,21 +16,6 @@ how your use case cannot be satisfied properly using a workaround.
 
 In March 2026, we drop support for all c-ares versions before 1.16.0.
 
-## OpenSSL-QUIC
-
-OpenSSL-QUIC is what we call the curl QUIC backend that uses the OpenSSL QUIC
-stack.
-
- - It is slower and uses more memory than the alternatives and is only
-   experimental in curl.
- - It gets little attention from OpenSSL and we have no expectation of the
-   major flaws getting corrected anytime soon.
- - No one has spoken up for keeping it
- - curl users building with vanilla OpenSSL can still use QUIC through the
-   means of ngtcp2
-
-We remove the OpenSSL-QUIC backend in January 2026.
-
 ## RTMP
 
 RTMP in curl is powered by the 3rd party library librtmp.
@@ -71,3 +56,4 @@ CMake 3.18 was released on 2020-07-15.
  - Support for Visual Studio 2008 (removed in 8.18.0)
  - OpenSSL 1.1.1 and older (removed in 8.18.0)
  - Support for Windows XP (removed in 8.19.0)
+ - OpenSSL-QUIC (removed in 8.19.0)

docs/HTTP3.md

@@ -250,54 +250,6 @@ Build curl:
  If `make install` results in `Permission denied` error, you need to prepend
  it with `sudo`.
 
-# OpenSSL version
-
-QUIC support is **EXPERIMENTAL**
-
-Use OpenSSL 3.3.1 or newer (QUIC support was added in 3.3.0, with
-shortcomings on some platforms like macOS). 3.4.1 or newer is recommended.
-Build via:
-
-     % cd ..
-     % git clone -b $OPENSSL_VERSION https://github.com/openssl/openssl
-     % cd openssl
-     % ./config enable-tls1_3 --prefix=<somewhere> --libdir=lib
-     % make
-     % make install
-
-Build nghttp3:
-
-     % cd ..
-     % git clone -b $NGHTTP3_VERSION https://github.com/ngtcp2/nghttp3
-     % cd nghttp3
-     % git submodule update --init
-     % autoreconf -fi
-     % ./configure --prefix=<somewhere2> --enable-lib-only
-     % make
-     % make install
-
-Build curl:
-
-     % cd ..
-     % git clone https://github.com/curl/curl
-     % cd curl
-     % autoreconf -fi
-     % LDFLAGS="-Wl,-rpath,<somewhere>/lib" ./configure --with-openssl=<somewhere> --with-openssl-quic --with-nghttp3=<somewhere2>
-     % make
-     % make install
-
-You can build curl with cmake:
-
-     % cd ..
-     % git clone https://github.com/curl/curl
-     % cd curl
-     % cmake -B bld -DCURL_USE_OPENSSL=ON -DUSE_OPENSSL_QUIC=ON
-     % cmake --build bld
-     % cmake --install bld
-
- If `make install` results in `Permission denied` error, you need to prepend
- it with `sudo`.
-
 # `--http3`
 
 Use only HTTP/3:

docs/INSTALL-CMAKE.md

@@ -275,7 +275,6 @@ target_link_libraries(my_target PRIVATE CURL::libcurl)
 - `ENABLE_UNIX_SOCKETS`:                    Enable Unix domain sockets support. Default: `ON`
 - `USE_ECH`:                                Enable ECH support. Default: `OFF`
 - `USE_HTTPSRR`:                            Enable HTTPS RR support. Default: `OFF`
-- `USE_OPENSSL_QUIC`:                       Use OpenSSL and nghttp3 libraries for HTTP/3 support. Default: `OFF`
 - `USE_SSLS_EXPORT`:                        Enable experimental SSL session import/export. Default: `OFF`
 
 ## Disabling features