Man pages matching name ssl_conf_ctx_set_flags in section 3ssl:

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