When syncthing is configured to go both ways (the default), it also syncs any deletions. You can somewhat get around this by something like one way sync, but it's not really a proper "backup" software.

Personally, I like to treat data synced by syncthing, even between multiple machines, as one copy of the data when I am following the 3-2-1 backup rule*, because syncthing won't save me from a buggy program deleting all my files or user error, or anything like that.

*See wikipedia for info about the 3-2-1 backup rule.