Someone earlier mentioned wanting the underdark. And we got the underdark in Out of the Abyss. There's a ton of generic Underdark setting material in that campaign. Which is how I think they're going. rather than republish all old material that's perfectly compatible with 5e again, they're combing the changes into a campaign path adventure
Oh, nvm. You typed Undermountain, so I thought you meant Halaster's dungeon under Waterdeep, not the Underdark.