I did look into it. By default, when the folder on the phone is set to "Send Only" and the backup device is "Receive Only", deleting photos on the phone will also delete them on the backup after syncing.
You'd have to move the files in the backup to avoid deleting or configure the backup folder advanced options to "ignoreDelete". However, not guaranteed to work. There are posts on the Syncthing forum around the same question, e.g. Send and Delete.
I doesn't work like that as that's not the intended purpose of Syncthing. It's a drive/folder mirroring software not a backup solution.
I wanted to use it to backup photos from my phone into a hard drive. When the photos are deleted on my phone, Syncthing sends a delete command. The send and receive options in Syncthing are for commands but delete is also a command that is sent.
Rsync is likely a solution to incremental copy of files from a phone to a NAS where you can delete locally without impacting the NAS backup.
Even those that aren't residents and happen to be visiting or on a visa.
“Our society must make it right and possible for old people not to fear the young or be deserted by them, for the test of a civilization is the way that it cares for its helpless members.”
Here's the thing, it's already paid for from your taxes. Some countries choose to feed the military machine where a few billion lost here and there isn't an issue. However, when it cones to healthcare, it's an insurmountable cost.
Know what the best thing about national healthcare service? I don't have to think about it. It's there. How much it costs? Doesn't matter? Will I become destitute if severely ill? No. Do I allow my fellow countrymen to be supported in times of need? Absolutely. Do I obtain natural hedging benefits from pooling risks into a bigger pot rather than having broken risk pools managed by thousands of insurance companies? Yup. Do countries with national service have greater costs than for-profit insurance healthcare? Nope, quite the opposite. Is life expectancy shorter in countries with national service? No, on the contrary, people live longer.
Making sure everyone has access to treatment when in need should be viewed as a large achievement of societal living and a great patriotic duty, your fellow citizens are protected.
But sure, without insurance middlemen milking people dry some people won't get to enjoy their third mansion. Well, only if someone doesn't have preexisting conditions otherwise you're on your own and left to get treatment when in need and filling bankruptcy at the same time. The pinnacle of human development.
I'd try the "ignoreDelete" advanced option in the backup device and test if deleting files in the phone maintain them in the backup device. Note the recommendations in the documentation. For my needs I decided to manually rsync to a NAS.