It's definetely not possible to put folders on dock in iOS 9 (not sure about iOS 10 though).
I will be using this feature in iOS 11 for easy access when using split view.
Edit: ^ Sorry my mistake - you can put folders on the dock in previous versions of iOS.
Ok sorry it does work in iOS 9. Silly me I was trying to load folder onto dock when I have already reached the limit.
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I have a question about the behaviour of folders on the dock in iOS 11.
When you are in an app and activate the dock by swiping up so you can initiate split view, what happens when you press on a folder? Does the folder hover above the app and allow drag and drop for split view or does it kick you out of the app and display the folder on the home screen?