With a small online population that has been "peaking consistently at approximately 20 players or less" over a number of months, 343 Industries yesterday announced via the Halo Waypoint Blog
, that the multiplayer servers for http://www.trueachievements.com/Halo-2-PC/achievements.htm
will be turned off on February 15th.
At the moment, it isn't clear if the server closure will have any impact on the game's online achievements, as multiplayer will still be available over LAN.
For all of you completionists though, I'd strongly suggest you dust off your copy and earn whatever achievements you can before the closure date, just to be on the safe side.
Thanks to all of the community members that sent in this story!