Not sure if this is official, but apparently Halo 2: Anniversary is now available in The Master Chief Collection for PC via Steam.
Taken from TPU … Microsoft Game Studios and 343 Industries have released halo 2: Anniversary for the PC platform. Being distributed as a package via The Master Chief Collection (which already includes Halo: Reach and Halo: Combat Evolved Anniversary), the game takes you through the second installment in the Master Chief’s saga to protect humanity from the Covenant. The game has been fully optimized for the PC platform, as have previous releases.
The second installment in the sage features dual-wielding, as well as a dual-character campaign where you’ll swap between the POV of the Master Chief and the Arbiter, a Sangheili commander fallen from grace in the wake of the events of Halo: Combat Evolved. The new Anniversary remaster of Halo 2 is a remaster in more than graphics, but in spirit as well, featuring added prologue and epilogue cutscenes, in-game terminals for you to bask in the glorious lore of the Halo universe, and reinterpreted cutscenes made by none-other than the amazing Blur Studios. The game and its contained updates for the Master Chief Collection will come in at 41.5 GB. Halo 2: Anniversary is available stand-alone for €9.99, or as part of The Master Chief Collection, which for €39.99, also includes Halo 3, Halo 3: ODST, and the storytelling masterpiece that is Halo 4.
Source: TPU, available on Steam