"GrapheneOS is the only mobile OS I can think of that lets you disable networking on a per-app level."

Don't need to "root" mobile phone and install GrapheneOS. Netguard app blocks connections on a per-app basis. It generally works.

But having to take these measures, i.e., installing GrapheneOS or Netguard (plus Nebulo, etc.), is why "mobile OS" all suck. People call them "corporate OS" because the OS is not under the control of the computer owner, it is controlled by a corporation. Even GrapheneOS depends on Google's Android OS, relies on Google hardware, makes default remote connections to a mothership that happen without any user input (just like any corporate OS), and uses a Chromium-based default browser. If one is concerned about being tracked, perhaps it is best to avoid these corporate, mobile OS.

It is easy to control remote connections on a non-corporate, non-mobile OS where the user can compile the OS from source on a modestly resourced computer. The computer user can edit the source and make whatever changes they want. For example, I use one where, after compilation from source, everything is disabled by default (this is not Linux). The user must choose whether to create and enable network interfaces for remote connectivity.