I think you have to label this as a bug as it was clearly an oversight in the patch notes to nerf a champion without meaning it. I would hope when a champion is nerfed it's based on lots of statistics and discussions between a mix of professional and soloQ games. It doesn't happen on accident or without telling us.

Now for this invincibility change, is would have been fine if you actually intended that and told us about it, but as a player I get patches thrown at me every 2 weeks. I'll be playing SoloQ, now something I'm positive worked in the past is no longer working, or maybe something that wasn't working is now working. I play Twitch a lot and a similar example is his ultimate. So first of all, his ult, so when I first played him his ult worked randomly on towers/inhibs. I started in the middle of Season 3 sometime. Then eventually it wouldn't work at all, and one day it worked perfectly. Then I believe during the rengar Q bug fix/tower changes Twitch's ult stopped working again. I actually lost a game trying to backdoor, I remember it and went to check the patch notes and I see mentions of how Rengar's Q won't hit tower anymore, but nothing about Twitch. I even mentioned it on forums a few times and the usual responses were "Backdoor ult is OP, so Riot nerfed it." But you didn't nerf it, you accidentally bugged it.

Now, however many months later, Twitch can hit buildings again. But I still don't believe there was any mention in all the patches it was randomly changed, maybe in the final one (where it was fixed for good) it may have been mentioned.