Halo: CE (i.e, Halo 1) didn't support online play, as it was released before Xbox Live was available.
Halo 2 was released in 2004 (on the Xbox!), and the online play servers were just shut down earlier this year. Given that the platform it was released on has nearly been superseded twice by now (first by the Xbox 360, and soon by the Xbox One), I think they've given it a fair run.
As far as I'm aware, all of the other Halo games are still playable online.
Wikipedia has Halo 2 multiplayer on the xbox being shut down in April of 2010 (while on the PC it lived until this year). That is not the sort of lifecycle I expect from a game that I enjoy (I still play Doom on occasion, and that was released two whole decades ago. Half-Life, from 1998, is still played online as well. Halo 2 was only from 2004, was massively successful, and is dead.)
Halo 2 was released in 2004 (on the Xbox!), and the online play servers were just shut down earlier this year. Given that the platform it was released on has nearly been superseded twice by now (first by the Xbox 360, and soon by the Xbox One), I think they've given it a fair run.
As far as I'm aware, all of the other Halo games are still playable online.