Comment 2 for bug 1609864

The correct link seems to be https://github.com/GoogleCloudPlatform/compute-image-packages/blob/fa80cdb23df0a7e190e228a9d9d379922d1f0764/packages/google-compute-engine/src/lib/systemd/system/google-shutdown-scripts.service

AFAICT, the way most services do this is to have the command you want to run in ExecStop, then have RemainAfterExit=true which means that the service is always considered active and thus ExecStop only gets run when the system is shut down.

This seems a reasonable feature request for snapd, but admittedly is probably low priority right now.