I have a tendency to learn stuff in reverse. I used to make an SD Card ready, connected monitor and keyboard and enabled everything as needed. Then I got the task to deploy n-RPi's and looked at the image first to customize it for the requirements we had. That's when I looked around and saw the unusual unit file and tried to understand why the service would look at the boot partition to start a service. In the end I think I added the symlink to start the service thorugh systemd.

I don’t know the details of how they implement this but it sounds like you want a systemd drop in file to override the ConditionPathExists (or similar) directive (I’m assuming you can’t or don’t want to modify the upstream unit directly given the use of symlinks).

It's at https://github.com/RPi-Distro/raspberrypi-sys-mods/blob/mast...

Or you can chroot to the mounted Raspbian root partition and do a normal `systemctl enable ssh` as part of your image customisation. Because, to be clear, you do not have to put a file in /boot to enable SSH, as it was claimed above. That is purely a helpful shortcut.