Unlike the discontinuation of Xbox Live in April 15, 2010, the PC has a few more options. Some games have been ported to Steamworks by their original developers, other games can be directly connected by a console command, and Halo PC can now be attached to GameRanger. Sure, it is a third-party application, but it functions. For fans of Halo, GameRanger also supports Halo: Custom Edition for gamers who want mod support. It even supports the demo for some reason.
I can still play online Halo 1… Anniversary Edition… until 360 Xbox Live is shut down too.
(Note: Yes, Halo: Combat Evolved for the original Xbox *never* had online.)
Sure, it would be better for the platform to evolve in the direction of true perpetual support. We can all hope for, and work toward, that future. Also, we can do that while sniping drivers out of Warthogs.