The release date was confirmed during X019.
Halo: The Master Chief Collection

Microsoft announced today that Halo: Reach will be released on PC come December 3. Also on this date, the game will of course also be added to the Xbox One release of Halo: The Master Chief Collection. While this may be one of the final pieces of content for the Xbox version of Halo: The Master Chief Collection on the Xbox, it will be the first piece of content out for the PC version.

Halo: The Master Chief Collection was announced earlier this year for the PC. Unlike the initial release on the Xbox One, which turned out to be disastrous on multiple fronts, 343 Industries wants to ensure that the PC release isn't terrible. As such, they are taking their time with each title, getting them out in chronological order. That's also why Halo Reach is first for the PC release.

With today's announcement, you can now also purchase Halo: Reach from the Microsoft Store or through Steam for $39.99 (USD).