supercar? not really in my opinion:
In my understanding of the term "supercar" (which I will group Hypercars and Megacars into for these purposes), it needs to tick a few boxes:
It needs to be visually stunning
It needs to be fast
It needs to be rare, which in turn, makes it very expensive
The Golf R is not 'visually stunning'. It's attractive, it's handsome, it's understated. It does not grab your attention in a parking lot, going down the road, etc.. it might stand out to car guys, but it's not a 'look at me car'.
The Golf R is fast. Actually, it's supercar fast in many respects - as fast as many supercars of not-too-far-back vintage, and with very light modifications, it will actually hold up well against many modern supercars too.
The Golf R is not truly rare. It's not expensive. For a Golf, yes, it's pricey. But it's not a 'dream car' in the true sense. A supercar is typically very expensive. $200k+ (yes there are exceptions, but they are just that - the exception) I've been in and driven supercars. I've never been in one of them and pulled up next to another one at the light. I've only had my R a little more than a week and I've already done exactly that at least 5 times. And I live in a relatively small town. The final edition Viper ACR I saw the other day? That's a supercar. The R I was driving? no.. not really.