The configuration and behavior of critical notifications differ between iOS and Android.
I just got a large batch of water leak sensors and was setting an automation to send a notification to my phone and I was worried that I would miss the notification. I found this page in the documentation for doing critical alerts. On iOS the notification stays persistent, even after unlocking the device. What’s really nice is that if you have an Apple Watch, the notification will also show up there and play a sound if the watch is on silent mode.
Even though I hope I never get one of these notifications, I’m really glad that this functionality exists within Home Assistant.
At least on Android, you can make TTS go through your alarm channel on your phone at max volume, to really make sure you don't miss it. I do this for my alarm and doorbell (only when I'm at home).