I have looked into developing a PWA. For starters:

1. Decent storage API. Last time I checked, there were serious limitations on the amount of storage you can use, especially in iOS

2. Mechanism for the user to save a certain . Analogous to saving and running a .exe and being able to compare the file hashes and run different versions of a file without the app developer's intent. This would include the ability to write and edit web apps from your device.

3. Some way of sending TCP/UDP packets directly, and doing port forwarding through UPnP.

4. Mechanism to run processes in the background, and for inter-app communication.

For example, you could not make a decent bittorrent app as a PWA. This is an example of an app which is prohibited on the app store despite having Apple having no legal basis for doing so.

I've often wondered if it would be possible to make some sort of "PWA Browser" that would give web apps hooks to some of this functionality, but it would probably get banned (There are no real rules on the app store, they can just ban you for whatever they want).

And this has nothing to do with my question - what features are lacking that would prevent this particular app from being a web app?

Looking at the changelog, ICEBlock uses push notifications, background jobs, and networking. https://www.iceblock.app/

There's also mention of a GPS fallback which doesn't sound like something you could do in a PWA.

PWAs can’t use Sockets, for one thing.

What on earth would you need that for in this instance?

As a side note, when Apple finally gets their shit together WebTransport is excellent for most socket like use cases.

web transport requires HTTPS, which requires a domain name and PKI for everyone you connect to