1.3.9 has been released a while ago: https://serf.apache.org/abi/changelog/serf/1.3.9/log.html Changelog mentions support for OpenSSL 1.1, so this will likely fix #592508 as well.
In repository via https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=03ff19afffb33c3d56336409ec06af59c9ecc4b6