The most likely attack model I can imagine is that a jailbroken phone still won’t be able to violate certain functionality (eg a recording LED remains lit, various supervisor functionality can’t be disabled, etc)
(Though I’m not convinced that will actually work on modern apple devices, where the led is pixels that run through the compositor — I guess the video driver stack and window managers are also exclaves in this world?)
Once you have that, you can take the idea further: Displaying an indicator that confirms that all your keystrokes are going to an exclave validating your password, for example.
The much-hated touch bar actually enabled just that, for Apple Pay payments, as far as I remember: It could display something like "touch to confirm payment of $x" on its own screen in a way that was impossible to manipulate from macOS – now here's an opportunity to bring that level of security back without requiring a dedicated display or taking away people's beloved function keys.
I would expect that to mean they're not included in screenshots so I'm curious now whether that's true for the iPhone 16.