Are you referring to untrusted devices? That is different than e2e encryption. Synching is e2e encrypted by default. Synching relays are only used when a direct connection with a peer is not possible. When a relay is used the data is encrypted. The relay cannot read it.
Furthermore, it prevents its use as a public bulletin board where the server can share a read key and allow anyone to keep a copy of the data. I am developing an E2E evoting app where that would have been useful.
You can also define that one folder on a given device (say a server) never accepts remote changes from anyone else, or never propagates local changes to anyone else. Combine this with encryption (provided by syncthing if you so desire, or your own) and versioning (also provided by syncthing if you want) and you have a pretty good backup endpoint.
I invite you to try out syncthing deeper, the doc really doesn't do it justice. However keep in mind that it's supposed to be used on devices that will be manually deployed. It's not made for large scale deployments.