Current Status:
This bug is fixed in openssh upstream and in Debian 9.10.
References:
- Debian bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=905226
- Phabricator bug report: https://discourse.phabricator-community.org/t/timeout-with-phabricator-ssh-hook-in-cluster/1205/2
- Patch against the debian openssh-server package: 905226.patch