SyncThing does support end-to-end encryption, and has for a while: https://docs.syncthing.net/users/security.html#sync-connecti...
By end-to-end, I meant that the server can't see your data. For example, the idea of deploying one or more SyncThing instances to cloud providers that would never see your unencrypted data. There's an open GitHub issue about it since 2014 (https://github.com/syncthing/syncthing/issues/109).
The only major drawback seems to be that you have to host the physical hardware yourself due to lack of solid end-to-end encryption for most platforms. Sandstorm might have it (I'm not clear on what's client- and what's server- side there), Seafile has end-to-end encryption that doesn't protect metadata (https://forum.seafile.com/t/how-strong-is-the-encryption/627...), and NextCloud appears to have a long-running beta of end-to-end encryption on a per-folder basis (https://nextcloud.com/endtoend/). Apparently Cryptomator exists (https://cryptomator.org/) although I've never tried it myself.
If you want to decentralize your shared files unfortunately last I checked SyncThing didn't yet support end-to-end encryption. (I don't think any of the other ones I mentioned can be used in a decentralized manner but things move quickly so I'm not sure.)
Alternatively, if you were thinking more chat and messaging there's self hostable federated services such as PixelFed, PeerTube, Mastodon, and Matrix. Or was there some other usecase you had in mind?