Hacker News Reader

Something that doesn't delete a file that I accidently deleted at source.

I have my syncthing configured to not do that.

What if the file is wrongly modified ?

You can configure Syncthing to keep X versions of a file for N amount of days, etc.

I don't disagree that Syncthing is not ideal as a backup solution but it can be a pretty decent one depending on your use-case.