Following Halo: The Master Chief Collection's announcement for PC, 343 Industries revealed that those who sign up for the title's Insider Program would be able to test early builds of the included games' PC versions. While these testing flights were projected to go online in April, those plans ultimately fell through.
Now, we have a new testing time frame to look forward to. 343 Industries community director Brian Jarrard recently said on Reddit that "currently it doesn't look like public flighting will begin prior to E3," adding that although the development team is making good progress, some areas are taking "longer than initially anticipated."
"Sure, if we could go back in time, knowing what we know now, we probably would've waited to make the announce[ment]," replied Jarred elsewhere to a comment inferring the PC version was announced too early:
"We had plans and estimates and goals - including a need to get the word out so we could allow ample time to build up a Halo Insider program. We all expected flighting to have started by now but unfortunately some areas of development have proven to be more time consuming than anticipated."
He also reaffirmed that Halo: The Master Chief Collection's PC development "has zero overlap," with Halo Infinite. "I'd argue it's more challenging dealing with decades old game code with multiple intertwined titles and systems than building a new experience on top of a new custom designed engine and toolset," he explained.
Splash Damage and Ruffian Games are helping 343 with the PC version's development, and a newsletter by the former studio revealed that all games in Halo: The Master Chief Collection are scheduled to land on PC in 2019, starting with Reach. We will have to wait and see if the bumps in development will hamper those plans.