Yeah, I was actually aware of that, while writing the above. The AndroidManifest.xml is zipped into the APK-file, so even for closed-source apps, you should be able to check it.
Problem is, of course, that it doesn’t help less techy folks, but also that you can’t prevent app updates from suddenly adding internet access.
And that you can’t take it away from apps that do claim to need it. At some point, I had some sort of root/XPosed/whatever setup, where I could take this permission away from apps, but because this was a thing that couldn’t happen normally, they all just flopped over sideways, saying things like “Please connect to WiFi 🥺”.
It feels like a gimmick, is the thing to me.
I’m sure, there’s going to be tech enthusiasts who will love it, but as a techie, I’m just imagining all the glitches and whatnot that hastily-made, non-standard Android modifications will cause on each of these manufacturers’ phones.
Or even just something like unfolding your phone, the app realizing that the screen size changed and it just forgetting the current UI state, because well, need to redraw the UI. Android’s UI framework has a talent for that, although I don’t know if that’s generally fixed by now.