The MacOS X dock, since day one, has been able to be pinned vertically on either side or centered horizontally on the bottom, as a user option. It functions identically regardless. The dock was derived straight from the Rhapsody dock, which was just a port of the NeXTSTEP dock.

The RiscOS shelf thing was a simpler device, but it was a year earlier, agreed. But your claim was that MacOS X's dock is derived from CDE, or I guess now RiscOS, and these claims are very highly dubious. Indeed with a good probability NeXT's engineers had not even seen RiscOS. The NeXTSTEP dock, complete with a black hole (later recycling sign) for trash can, was meant to be reminiscent to how people often laid out icons on the Mac Finder Desktop.

The only one I remembered doing that did that by default is CDE, not NeXT. Someone else brought up RiscOS.

By your logic, I could say Xerox Alto was the first if a launcher could be added and centered at the bottom manually :)