Doom is a bit of a mess on Steam right now. You have Doom 3, Doom 3: BFG Edition, Doom 2, Doom 2 (Enhanced), Master Levels for Doom 2, Final Doom, and more—all separate. So id Software is merging certain games into packages and bundles for "an improved shopping experience."
If you buy Doom 2, you'll get the aforementioned enhanced edition, master levels, and Final Doom, while if you have Doom 3, you'll be given Resurrection of Evil and the BFG Edition. Existing Doom 3 owners will get the upgrade and DLC for free starting today. And no games are being removed from the store or libraries as you'll be able to launch each separately once you've purchased a bundle or package.
The first Doom is seeing the most simple change. It's being renamed from Ultimate Doom to Doom (1993) to avoid confusion with Doom (2016). Then there's Quake 2 which is now bundled with The Reckoning and Ground Zero, while Quake 3 Arena is paired with Quake 3 Team Arena.
As for bundles, there are three—the Classic bundle gets you Doom (1993), Doom 2, Doom 3, and Doom 64. The Doom Franchise bundle gets you Doom Eternal Deluxe Edition, Doom (2016), Doom (1993), Doom 2, Doom 3, and Doom 64. And finally, the Quake Collection bundle gets you Quake, Quake 2, Quake 3 Arena, Quake Live, and Quake 4.
As for delisted titles — you can't find Quake 2, 3 Arena, or 3 Team Arena's demos on the Steam store anymore. 3 Team Arena has also been delisted but this will now be included with 3 Arena. Final Doom and Master Levels have also been delisted but these now come with Doom 2. Resurrection of Evil and Doom 3: BFG Edition have been delisted but they now install with Doom 3.
The Doom Classic Complete bundle is the last to get delisted but it is being replaced by the Doom Classic bundle, while Quake 2 Mission Pack: The Reckoning and Ground Zero are no longer available, instead being added to the Quake 2 launcher. This should make things simpler for those trying to get into Doom and Quake.