Man pages matching name ssl_conf_ctx_set1_prefix in section 3ssl:

SSL_CONF_CTX_set1_prefix.3ssl.en	debian 10 / libssl-doc
SSL_CONF_CTX_set1_prefix.3ssl.en	debian 11 / libssl-doc
SSL_CONF_CTX_set1_prefix.3ssl.en	debian 12 / libssl-doc