[Sync For Backup - And No Deleting]

Ok, I know this is an old topic but the discussion pops up pretty high on a Google search. So here's my solution.

I have a Resilio node running on a Linux server, some machines send files for backup (i.e. they should not be removed from the server even if deleted on the work station). 
To preserve my files I use Watcher. https://github.com/splitbrain/Watcher
Watcher is then started with crontab (@reboot feature).
Watcher is monitoring the backup folder after move_to and write_close events. For Resilio, only move_to is needed. But with write_to also files manually moved to the server backup folders will be triggered.
When the event occur following is executed:

if [ -f $filename ]; then chattr +i $filename; fi

This means that the file becomes immutable. 
It also means that modified files will not be updated, that is okay as this is an image/movie backup and the file names or path will always be unique.

Attached is a job configuration example for watcher.

watcher.ini