It seems that Microsoft won't be pulling the plug on Halo 2 PC yet, as 343 Industries' David Ellis has stated that the multiplayer has been extended to June. Halo 2 PC multiplayer was to be pulled on the 15th February.
He also added that there might be "further support options," which seems to indicate that the "Halo games could come to Steam" is true. If so, Microsoft is probably looking for a way to have the Live Achievements compatible with Steam.
Halo 2 multiplayer on PC extended til June
13 February 2013 | By JonahFalcon