The fun part is that android.permission.INTERNET is something you have to declare in your app's manifest, otherwise any and all networking for your app will be blocked by the system. It has existed since the very first Android version. This permission is automatically granted by the system upon app installation. So the only thing missing is this permission being a runtime permission that the app has to request and user has to grant.

This is available in iOS - but for whatever reason, on devices outside of China, you are only allowed to disable use of cellular data (not WiFi).

This is also available in some Android skins (OxygenOS is one, for example) - but it is not in AOSP.

Absolutely. Currently I'm using solutions like Blokada for that.

There are controls to disable internet access on a per-app basis. Seen them in devices from at least two different vendors, one of them being Pixel - thus I believe this is a stock feature on at least Android 10+.