Whether it exists yet or not, I’m certain it will at some point. It’s such a huge part of the game, really, that tongue-in-cheek ongoing gag, there is no cow level, but there absolutely is. I think the game devs want people to play all the other cool stuff in 4 first, before they introduce something that will inevitably tear people’s attention directly away from it.

I think the same things happened in Diablo 3, there’s no cow level until it’s specifically added in some patch (the zone is called “Not The Cow Level” btw)