Syncthing by default is set to sync multiple folders so they get to the exact same set of files and folders. So if you delete a file in one folder, the deletion is propagated. In the contexts of backups, this could cause issues. Examples:
Ransomware on any device: Then, all the devices would receive the corrupted data which replaces the valid data.
One of the devices runs out of space, and its user decides to clear out old backups.
The solution is:
On the device sending the backups: set Folder Type to “Send Only”
On the device(s) receiving the backups: set File Versioning. Recommended setting is “Staggered”
I would not recommend trusting syncthing’s versioning for any critical data. It’s a nice simple option to likely retain remotely deleted/changed data, but it’s by no means robust/tested enough that I’d trust it for backups.
tl;dr: Please don’t use this method as your only means of backup.