The notification reminder feature on HTC 10 notifies you of unread notifications repeatedly at given interval. To use notification reminder on HTC 10 and turn on / turn off repeat notification, do as follows:
1. Open the Settings app on your HTC 10
2. Scroll down to the Personal settings and touch to open the Accessibility settings.
3. Tap to open the Unread Notification Reminder settings, under the System settings (Just tap the text, not the toggle button)
4. Tap the Reminder me Every option to set the Frequency. (from every 15 seconds, to 5 minutes)
5. Now, toggle on the Unread Notification Reminder feature and from now on, your phone will remind you of any unread notification at the specified interval.