Disclaimer: I am not a synapse dev, but I have run a non-federated server for a long time.
Assuming you have fully disabled federation and Matrix.org provided services, data should not be leaving your instance. I run mine with bridges for services like Signal, IRC, and Discord, so as expected data goes out through the bridge software (via VPN).
To be extra safe, you could run it in a Docker container who’s network is limited to local, and an nginx proxy for Element.
Disclaimer: I am not a synapse dev, but I have run a non-federated server for a long time.
Assuming you have fully disabled federation and Matrix.org provided services, data should not be leaving your instance. I run mine with bridges for services like Signal, IRC, and Discord, so as expected data goes out through the bridge software (via VPN).
To be extra safe, you could run it in a Docker container who’s network is limited to local, and an nginx proxy for Element.