Even after disabling crash reporting, auto-update, relays, discovery, etc, it will still connect to the internet. It's actually very difficult to make it behave.

The only solution is to blank a few URLs in the configuration for things that can't be disabled. Then at first sight it seems to work... But you check your firewall logs a few days later and what do you know? Some dropped connection attempts.

Still, there aren't many alternatives that work as well as SyncThing, so you gotta pick your battles...