I can’t argue with personal preferences, but keeping your notifications grouped by app can be very confusing if you just want to know which ones were the latest received.
The Notification Center is the place where you can manage your notifications and while it’s not a new feature, it did receive some improvements in iOS 9. Now you can easily change the order in which your notifications are displayed and among the provided options, there’s one that lets you set them by date. Here’s how to do it:
Open your Settings app and tap on Notifications.
There you have two settings for sorting them: Sort Order and Group By App
If you want to see a unified list of notifications from newest to oldest, make sure the Group By App option is disabled then tap the Sort Order option, and select Recent.
For me, it just makes more sense to see all notifications in the order they arrive using the settings above. However,you could keep Sort as Recent and group your notifications by app as well, but you’d need to remember which app notified you and scroll to its group to see the newest notification.