Halo 2 was the last of the mainline entries in the series to get a PC release back in 2007, just a few months before Bungie finished Halo 3 for the Xbox 360. As part of the effort to bring The Master Chief Collection to PC, 343 Industries says Halo 2, in its Anniversary form from 2014, will go into testing as early as late March, suggesting the port's release may be only a few weeks or months away.
In an official forum post, 343 says that in addition to Halo 2 and Halo 2: Anniversary, the Forge and Theater modes for Halo: Reach will also be part of its next "test flight" period of early access for Halo Insider members. "[I]f all goes well, we are targeting our next public flight for the end of March," the post reads. "If we remember back to earlier this year, we aimed to start [Halo: Combat Evolved] flighting in January, but it ended up rolling into February when those kicked off. Please be patient as we want to get these out as quickly as possible, but in development: things happen."
The turnaround between the start of testing for the new PC port of Halo: Combat Evolved and its official release was incredibly fast. After a single two-week period of testing in February, the port was released as a surprise on March 3.
Neither Halo: Combat Evolved or the version of Halo: Reach that launched for PC in December are without their problems, but 343 says it is committed to continuing bug fixes and other patches for both.
At this rate, if Halo 2 comes to PC in March or April, it's possible we could see the complete Master Chief Collection on PC ahead of the release of Halo Infinite later this year. After Halo 2's port is released, 343 will move on to bringing over Halo 3, Halo 3: ODST (only the campaign), and Halo 4.
If those ports are all released ahead of Infinite, then the entire mainline Halo series will be playable on PC save for 2015's Halo 5: Guardians. Infinite will launch for the Xbox One, Xbox Series X, and PC later this year, timed to release alongside the Series X.