hashtags are used because older browsers don't let you use pushState. pushState is used because it's more correct, and doesn't exploit a hacky loophole.
Both are used because they are faster than a whole new page request, and they let you share the post by cutting and pasting the URL in your address bar.
But it's not without its idiotic blunders. Topping the list is Apple's insistence that your user ID be an E-mail address. That's not just a bad idea, but it has resulted in untold numbers of people with numerous Apple IDs and content bought with all of them.
I actually think this is a good idea. An email address is unique and easy to remember for the user.
The stupid stuff is the following:
This was not always required. I think there was a transition period were it was not enforced but only advised on the register page. And also somewhat ambigous like "Enter here your Apple ID username eg. BruceWayne@mac.com" I know two people who entered their (company) name in such a way, but without really having a dotmac address! And then dotmac was changed to Mobile Me and even though me.com was retained for iCloud I couldn't use an "old" Mobile Me email-adress, which was sad because it was a nice address with the initials of my name, but had to make a new me.com iCloud account/address. Really Confusing.
That's the issue though. It doesn't introduce anything useful in terms of replacing JSON (it can even make things worse), and for other uses better solutions already exist.
The fact that in theory if it was magically supported everywhere, it might be serviceable is neither her nor there. It's not useful information. It's not gonna happen because there's no reason to switch from JSON to this, and many reasons not to. Likewise, for other applications there are better solutions.
People started using JSON even though at that point it wasn't standard because it had real benefits over what they were currently using. There isn't any situation where using this would provide a real benefit except to satisfy personal whims, and it would have the drawbacks listed.