Comment by AnthonyMouse

> When I open Slack, to which servers am I explicitly requesting a connection?

Debian only supplies open source software. Proprietary apps that only support the vendor's service aren't included as it is. Open source apps using standard protocols like Matrix or similar do allow the user to choose the server.

> A middle ground might be to create a distro that uses something like SELinux to prevent all network access to non-system processes.

We're talking about open source software in the official repositories. You're not putting it in a jail to thwart it from defecting on you, you're modifying the code so that it doesn't even try.

> How do updates work?

When you install Debian it asks you which mirror you want to use for updates. Several of them are provided by universities etc. You can also make your own and some large organizations do that.