My 2006 GTI has rubber around filling that gap. I thin VW stopped it on 07 models (assume cost cutting)
Perhaps it can be ordered and fitted to newer models
Nope, it comes as part of the window. I have had one of my rear side windows replaced due to scratches. The trim and rubber seal are all one part, ready bonded to the glass. There's no way you could retro fit it I'm afraid.
One solution is to buy a complete set of early GTI spec blue tinited glass from VW. Then you'd get the surround. Blue's better than the new green tint anyway