A non-app-store web app on iPhone has been able to be full screen since initial release of Home Screen web apps. When you launch from Home Screen, it gets the whole screen.
See the Xbox Cloud Gaming "app" for instance, which is outside the App Store, just launch then "Add to Home Screen", close, and run from Home Screen.
As for what can be done with browsers, see the venerable iCab but also Kagi's Orion browser which runs Firefox and Chrome extensions, even on iOS. Yes, it's WebKit based, but so was Chrome for a long time.
Given you can run Xbox games or arbitrary extensions from other browsers, it's clear the web app and WebKit limits are less restrictive than most discussion acknowledges.
For the last few features that used to be missing, like notifications or other native hooks, notice Microsoft has the sidecar native app for iOS that handles in-game chat, LAN discovery for Xbox setup, and notifications.
To be clear you absolutely cannot run Xbox games in a web browser. The service you’re talking about is just streaming video from a remote Xbox to the phone.
You and the OP are both right about fullscreen. There is a web fullscreen API, which Apple does not support. However, PWAs strip out the browser UI so you’re effectively fullscreen. Though you can’t do anything about the status bar, nor can you lock screen orientation.
But more to the original point, none of this has anything to do with security. Apple disallowed a native Xbox streaming app because they demanded a cut of the revenue and MS wasn’t willing to give it.
See the Xbox Cloud Gaming "app" for instance, which is outside the App Store, just launch then "Add to Home Screen", close, and run from Home Screen.
https://www.xbox.com/en-us/play
As for what can be done with browsers, see the venerable iCab but also Kagi's Orion browser which runs Firefox and Chrome extensions, even on iOS. Yes, it's WebKit based, but so was Chrome for a long time.
https://help.kagi.com/orion/browser-extensions/macos-extensi...
Given you can run Xbox games or arbitrary extensions from other browsers, it's clear the web app and WebKit limits are less restrictive than most discussion acknowledges.
For the last few features that used to be missing, like notifications or other native hooks, notice Microsoft has the sidecar native app for iOS that handles in-game chat, LAN discovery for Xbox setup, and notifications.