I don't know how long it has been a thing, but I just upgraded to 8.3 on my iPad Air that I've been neglecting lately, and decided to change around the icon locations. I mindlessly took an app out and placed it on my dock, which already had 4 icons, not thinking anything of it. It actually worked, which surprised me. I was so excited! Sorry if I'm late I wanted to share if this is a new thing! A quick Google search gave irrelevant results, so I figured it was a new thing. I also tried this on my iPhone 6 Plus which also has 8.3, and it didn't work. So it's iPad only.