I found resolvconf was missing on my system. Yet, openconnect is installed and set up to make use of resolvconf. I have not studied modify_resolvconf_generic() to see if resolvconf brings any value.
resolvconf support is optional. The ebuild already displays a message when you install openconnect. pkg_postinst() { readme.gentoo_print_elog if [[ -z ${REPLACING_VERSIONS} ]]; then elog elog "You may want to consider installing the following optional packages." optfeature "resolvconf support" virtual/resolvconf fi }