So, how does the latest iOS 11.3 public beta handle an iPhone crashing due to a worn battery? Well, I happen to have an iPhone in the lab that is awaiting a battery replacement, and after stress testing it for a few hours I managed to cause a crash.
On restarting, you will see a notification informing you that This iPhone has experienced an unexpected shutdown. Heading over to Settings > Battery will show you this screen:
Clicking on that Disable... link brings up the following:
Choosing to disable performance throttling cannot be undone, although the throttling will be re-enabled if the device crashes again.
What are the side-effects of disabling performance throttling? Put simply, it's the risk that your device will crash again in the future. And if the battery is worn, it's likely that it will happen again, with the probability increasing depending on how worn the battery is (the battery in the iPhone I crashed is at 82 percent of its maximum capacity and it didn't take me a long time to make it crash).
If you want access to the public beta, you need to sign up. You can do that here. Before you go any further, you might want to take a look at my list of precautions and caveats related to running beta software on your iPhone. While you can easily roll back from running the beta, it's possible for the beta to cause havoc in the meantime.
Note that this feature applies only to the iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE, iPhone 7, and iPhone 7 Plus.