Halo: The Master Chief Collection is still tentatively planned for release on PC in 2019 but progress towards its testing goals has been delayed in recent months. Originally planned for July, the test flight for Halo: Reach's Firefight Horde mode was delayed until "it's ready." It seems Firefight is ready now, as the next test flight for Halo: The Master Chief Collection has started.
343 Industries Engagement Coordinator Tyler Davis announced on Twitter that the "FireFlight" test flight would begin today. The plan was to send out invites to roughly 10,000 Halo Insiders for the test flight, pending final approval. Approval appeared to have been made, as Davis confirmed that emails went out at approximately 10:00 am PT.
The purpose of the current test is to verify the Firefight game mode and matchmaking for the mode at scale. In other words, 343 is testing whether things run smoothly on PC with a lot of people playing. All players have to do is join in the fun and play as much as possible. Three classic Halo: Reach maps will be playable, as will two playlists of Firefight. Beachhead, Corvette, and Holdout are the maps, and the two playlists include 4-player Heroic Firefight and 4-player Normal grab bag Firefight.
343 Industries goes out of its way to remind players who have been invited to the Halo: Reach Firefight test flight that this is a test, through and through. Technical issues are bound to happen, and 343 asks players to report the issues they discover as they're able. However, 343 also notes that the new Halo Support website is also being beta tested in preparation for launch.
While the test flight for Halo: Reach's Firefight game mode was delayed a month, 343 Industries has yet to say whether Halo: The Master Chief Collection itself has been delayed. Since the port's 2019 release date was only spoken by employees and not confirmed via an official announcement, it's entirely possible that the game has been delayed to 2020. Progress clearly continues to be made and the fact that 343 is testing online matchmaking is a hint that development is going strong. But these things take time.
343 has been aggressively transparent about the development of Halo: The Master Chief Collection on PC, so far. Prospective players should expect updates on future flights and the game's release in the relatively near future.