I didn’t know that was a possibility. Still, it seem kind of not really what Syncthing is intended for. I mean, they even state it in their FAQ:
No. Syncthing is not a great backup application because all changes to your files (modifications, deletions, etc.) will be propagated to all your devices. You can enable versioning, but we encourage you to use other tools to keep your data safe from your (or our) mistakes.