He's only 27. I don't think he has passed his prime. It's just that he's overrated.

People saw a ridiculously tall goalkeeper and assumed he must be a good GK. Or maybe he was really good at a certain style of save, but below average at the rest. It would make sense to me that he might be really good at stopping balls kicked into the corners from long range. It gives him time to get his big body moving, and his reach means he can get to balls that other keepers can't.

And, probably, those are more frequently the kinds of goals that Man City goalkeepers need to defend against. They dominate games and so other teams can't break them down and work the ball into the box, they have to gamble on long-range shots that gives Donnarumma time to adjust.

It's funny though, because I remember Guardiola pushing out Joe Hart because he was bad at distribution, and then Guardiola went for relatively short GKs who were good with their feet, like Claudio Bravo. It's funny that he later on switched to a gangly guy who was pretty bad with his feet, mediocre at claiming crosses, but a good shot stopper in certain situations.