That's not what happened at all. He didn't get nerfed, He actually got a visual update.
As for your experience, he didn't get nerfed, that is the sad result of him being left to become outdated. I mean, yes, a person can still play Alistar and they could get good results, I've gotten good results. And there's probably people who do well with Alistar. But to me, the unfortunate situation seems that Alistar is a champion you would have little reason to pick; It's seems that as a support, you could just pick another champion and get the same good results but with less effort.
But if anyone has positive ideas about him feel free to tell me; I would like to know.