In my time, I've played about 2000 games of Dominion online, and quite a bit IRL (a wild guess would be 500). I've run across actual pins in three of my games.
One of them was a designed kingdom with the pin in it. It was used in a tournament match and I was playing a bunch of tournament sets IRL. It was the KC+Goons+Masq pin vs. an Ironworks+Gardens rush. I went for the pin, he went for the rush, and I was one turn too slow and he won.
Here's the only online game, where I pull off a Cutpurse pin. Ghost Ship to discard, Masq to pass Coppers, and Cutpurse to decrease hand size. I got him to start a hand with one Copper when he resigned.
The other one was IRL and was once again a Cutpurse pin with masq to enable it, even though I don't even recall a handsize attack on that board.
So in my experience, the KC+Discard+Masquerade pin is less likely to show up, and it makes sense because KC+Masquerade are two specific cards, Cutpurse+Masquerade are two specific cards and I think there are more Villages than discard attacks.