I'm getting a little tired of running into decks running GvG cards. There's really no reason these decks should still be around. If you have a deck with cards that old, one of two things is probably happening. You either haven't played arena for almost half a year, or you're intentionally saving an absurdly powerful deck for some reason - usually ruining some streamer's day.
In the former case, just refund the deck and have them start fresh; they're probably not too attached to an arena run they started back in February and didn't finish. But I don't think this is the average case, because whenever I run into a wild Arena deck, it's usually the most crazy fucking shit I have ever seen. Which would more likely apply to the latter case.
In the latter case, you just shouldn't be able to do that. You're ruining the day of everyone you run into when you curve out Avenge -> Shieldbot -> Muster for Battle. Or the mage decks with multiple Flamewakers. And more to the point, the main reason people go through so much trouble to save broken-ass wild decks is because of, well, this. Which is not really okay. In fact, it's against Twitch's ToS, and I'd hazard a guess that it's against blizzard's code of conduct (falls under the fairly broad term "cheating") as well.
There should be some kind of timeout on these decks - 2-3 weeks after the rotation hits, if you have an active arena from before the rotation, it gets retired, or refunded, or something. I realize this is a problem that eventually will solve itself, but I bet that in a year or so we'll hear people complaining about running into decks with Meteor in them.