Halo 2 and Halo 2: Anniversary is now available as part of a public beta test if you've signed up for the Halo Insider program.
The first public test flight will be launched today, following a delay due to various technical issues. The news came from community manager Tyler Davis via Twitter.
PSA: All Ring 3 blockers have been resolved at this time for Halo 2 and Halo 2: Anniversary flighting to begin on Friday, 4/17. We will let folks know when buttons are being pressed for invites to roll out. Stay tuned to get tactical Marines!— Tyler “Postums” Davis (@343Postums) April 16, 2020
Much like the Windows and other Microsoft Insider programs, 343 Industries splits the testing across various 'rings,' with the third ring being publicly available for anyone to join. Halo 2 will be released to the third ring later today.
As such, if you want to give the game a try, you can sign up for the Insider program on the official Halo Insider website, although there's no guarantee you'll be able to take part in this first test. 343 Industries selects particular users from the third ring to take part during early testing, such as those with specific hardware configurations.