To reproduce this behaviour you will need to have several notifications on the Notifications Center - enough that there will be a “Clear All” button available on the heading just above older notifications.
- Ensure that VoiceOver is enabled.
- Either go to the Lock Screen or the Notifications Center.
- Move VoiceOver focus through the more recent notifications until focus is placed on the heading for older notifications.
- Swipe right with 1 finger to place VoiceOver focus on to the “Clear All” button.
- Ensure that the VoiceOver rotor is set to its Actions menu.
- Flick up with 1 finger repeatedly until VoiceOver says “Clear All”.
- Perform a 1 finger double-tap.
In previous versions of iOS this would clear all notifications.
Currently it does nothing.