Related game:
Halo: The Master Chief Collection (MCC)
As discussed here:
Is there a legitimate method for unreleased DLC unlocks?I'm not part of the Investigation Team, but I thought I should post this here to avoid any confusion in the future.
Many of the games in the collection are not currently available to play, yet there are some gamers unlocking those achievements before they are released.
As of 3 December 2019, only Halo: Reach is playable on the Steam version of MCC.
With all of that in mind,
achievement progress in MCC is transferable between Xbox One, Windows 10, and Steam versions as noted in the following support article on HaloWaypoint.com:
MCC PC Crossplay & Cross-ProgressionReporting any gamer for having unlocked achievements in games not currently available for play on the Steam version is just going to cause a major headache for the Investigation Team when they need to reverse any action made on those accounts. Please correct me if I'm wrong, thanks.