You don't need to audit the code to know if something sends telemetry. For many things (such as your kernel) you can rely on reputation/quickly searching through github issues. For others you can rely on packet sniffing.

I'm pretty confident that the only things I use regularly on my computer (not counting my phone here) that send telemetry are

- Firefox (opt out, but prompts you)

- Visual Studio Code (opt out, doesn't prompt you, requires re-opting out on updates)

- rustup (not sure if opt in or out, don't mind if it's recording telemetry)