I mean it clearly states in the news article that One for All is being removed on 12/2, but I've heard different, and that it will always be available in custom. I just need clarification.
I heard that they are planning to have rotating modes. And the reason they can't have all of them at once is server load. But I don't remember where I heard it so it's a pain to find a source, I may have even heard it in a chatroom so no "evidence".
I did hear that they were rotating official playlist like Freezman said, though I believe they did say that the newer playlist will remain available in custom game types. I'm not sure on the specific source though, perhaps it was an earlier Reddit?
