It comes as no surprise that people have been constantly asking about the release status of Halo: The Master Chief Collection on PC. It first had an extremely small test flight for Halo Reach at the end of June and is scheduled to another one at the end of July. However, it seems as though Microsoft might be prepping for the rollout sooner than expected.
According to SteamDB, it currently lists about 228 Steam achievements. All of these were added over the last week. Of course when talking about Halo: The Master Chief Collection, Microsoft already confirmed that they will be releasing it one title at a time for $9.99 starting with Halo: Reach. Most of the achievements listed do seem to be related to Reach so it makes sense. The latest I can see it being announced is during the Inside Xbox stream at Gamescom on August 19. Keep in mind that this is only a rumor at the moment but things do seem to be developing over at Microsoft and 343.