Not to discount the protein too much, but changing your routine (and I may be reading more into this than it actually is) after 2 years may have had more to do with your gains.
It's hard to separate. Last year I added lots of crossfit workouts to my schedule, switched up my 10 and 5 rep set full-body workout routine and bodyweight routine to a slightly shorter routine with all 3x5, and changed my diet. I gained some muscle and a lot of strength. Which was it that made the biggest difference? Hard to say, really. I knew that it would be a problem later but the shotgun approach was necessary...hit everything all at once and get results, where I got it from wasn't really important.
But protein powder is just food. Whey especially is fast-digesting so it's useful after a workout in a way that a steak isn't, but if you're trying to maintain I would think 150g of protein from chicken and steak and fish and 150g from those and whey protein is going to get you about the same results.