Man pages matching name ssl_ctx_has_client_custom_ext in section 3ssl:

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