I caught this in the XUbuntu release notes:

https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/21...

If anyone has a login to this system and can post an update, as an Arch + Xfce + gcr4 user a solution, one needs to add the unit `gcr-ssh-agent.socket` to the user units for systemd (i.e., /etc/systemd/user/sockets.target.wants/gcr-ssh-agent.socket ). This unit is provided by the gcr4 package and does exactly what they're discovering, sets SSH_AUTH_SOCK on login to match the running gcr-ssh-agent.