After it was announced in March that Halo: The Master Chief Collection is coming to the PC, the company said on April 2 that it planned to begin public testing by the end of the month. Today, 343 Industries said that now that we're at the end of the month, it can confirm that the project is delayed.
Moreover, it's not like it's delayed by a day or even a week. The forum post says that there is no updated timeframe. All it said was that the "343 publishing team and our partners at Ruffian and Splash Damage are working hard to get the pieces in place to support flighting as soon as possible."
It does seem like many of the PC parts are ready to go, as the forum post also says that "Reach is already pretty fun to play on a PC".
Halo: The Master Chief Collection includes the original four games including the Master Chief, including Halo: Combat Evolved, Halo 2 Anniversary, Halo 3, and Halo 4. Halo 3: ODST was also made available as an add-on for those that experienced bugs when the game first launched, and Halo: Reach is being added for the PC launch, although it's also coming to Xbox One.