Final PSA: If you own this on XB1 (it was GwG once) you can still download all the episodes before it will eventually be delisted. The game is now unavailable for purchase on PS3, PS4, X360 and Steam.
Let's hope 2K or Gearbox can relist this in the future because it was one of TellTale' best.
Sorry for the confusion, I am not 100% on if its not downloadble if you bought it before, but TrueAchievements mentioned this so thats why I'm saying it like this. Fact is that they are delisted so you can't buy it anymore.
"Once purchased, make sure you download all of the episodes to your console too, as several of the delisted titles are stopping players from downloading the season pass content."
Edit2: XB1 version also delisted now