The reports were right on the money, and Halo: Reach will release on December 3, 2019 on PC, including the Windows Store and Steam.
Today Microsoft confirmed Halo: Reach will arrive on PC as a full package this December, complete with Firefight, campaign, multiplayer, and huge PC optimizations like uncapped frame rates, 4K 60FPS support, and full mouse and keyboard support. We'll see playlists borrowed from the original 2010 launch including Firefight's online Score Attack mode as well as the classics like Team Slayer. It'll also have online co-op campaign too.
Reach is included with the $40 Master Chief Collection, but you can buy it separately on Steam for $10. The full Master Chief Collection obviously won't be available if you pre-order the complete package--only Reach will drop in December.