The Halo Online modded project known as ElDewrito recently received version 0.6 in its short development cycle. The mod enabled PC gamers to play the defunct version of Halo Online with other people... online. However, Microsoft wasn't too happy about that and there might be legal action taken against the project.
The game originally came out back in 2014 in partnership with Microsoft, 343 and Saber Interactive, but a couple of years later Microsoft had the project shut down. Some of the game's code ended up leaking out, and the ElDewrito crew began reverse engineering a multiplayer game around the available code from Halo Online. Well, after the project gained tons of popularity and began spreading throughout the PC community, Microsoft stepped in to claim that the project had to be shut down as a means of exercising its copyrights.
343 Informed the community that this doesn't mean that this is the end of Halo on PC, but right now the project needs to be put on hold...
Part of the issue that Microsoft had with ElDewrito was that it uses some of the Halo Online code, which is based on Halo 3 for the Xbox 360. 343 mentions that other Halo fan projects were fine because they were built on custom code and not any of the proprietary code belonging to Microsoft.
As noted by 343, Microsoft is still investigating servicing the PC community, just not right now.
One of the more popular entries in the series, Halo 3, was forfeited a release on PC. Microsoft attempted to compensate with the Russian version of Halo Online, but obviously gamers in the West and the East were unable to play it.
The ElDewrito mod for Halo Online opened up Halo 3's multiplayer for the PC gaming audience, giving gamers a taste of the action that Xbox 360 gamers had more than a decade ago. For now, Microsoft would prefer if people played Halo 3 through Halo: The Master Chief Collection, rather than reverse engineering abandoned projects.