I did this

Reading the manpage and trying to use options directly even though they were not documented (to see if it was just a documentation oversight), followed by reading the source code (to confirm whether I simply guessed the option name incorrectly) confirms that there is incomplete mirroring between the TLS options (--tlsv1, --tlsv1.0, --tlsv1.1, ..., --tls-max, --curves) and their proxy counterparts (--proxy-tlsv1, ...).

In fact, the latter option (--proxy-tlsv1) is the only one implemented, leaving you unable to e.g. instruct curl to use only TLSv1.3 when talking to an HTTPS CONNECT proxy itself.

I expected the following

It would be feature-complete to also have the following options:

--proxy-tls-max
--proxy-tlsv1.0
--proxy-tlsv1.1
--proxy-tlsv1.2
--proxy-tlsv1.3
--proxy-curves

curl/libcurl version

curl 8.4.0 (x86_64-pc-linux-gnu) libcurl/8.4.0 OpenSSL/3.0.12 zlib/1.2.13 zstd/1.5.5 c-ares/1.19.1 libidn2/2.3.4 nghttp2/1.57.0
Release-Date: 2023-10-11
Protocols: dict file http https mqtt rtsp
Features: AsynchDNS HTTP2 HTTPS-proxy IDN IPv6 Largefile libz NTLM SSL threadsafe TLS-SRP UnixSockets zstd

operating system

Gentoo Linux (amd64)