Man pages matching name curlopt_proxy_ssl_options in section 3:

CURLOPT_PROXY_SSL_OPTIONS.3.en	debian 10 / libcurl4-doc
CURLOPT_PROXY_SSL_OPTIONS.3.en	debian 11 / libcurl4-doc
CURLOPT_PROXY_SSL_OPTIONS.3.en	debian 12 / libcurl4-doc