Halo Online was co-developed by Innova Systems and Saber Interactive, the latter of which worked with Microsoft to upgrade the graphics for both Halo and Halo 2 for the the Xbox One game Halo: The Master Chief Collection. Here's how Microsoft described the game back in March 2015:
Halo Online is powered by a highly modified version of the Halo 3 engine and optimized for smooth performance on lower-end PCs. While there's no campaign mode, Halo Online is set on a secret UNSC space installation called Anvil, where Spartan-IV soldiers train together in war exercises to sharpen their battle skills and test experimental technology.
The game was in closed beta in Russia for several months, and at the end of December 2015, Innova Systems announced Halo Online was going to be "temporarily suspended". Now "Fogeyma" who claims to be on the game's development team, posted a note on the Russian social network VKontakte, (via GameSpot) stating that "the game in its current form will not be released."
Ironically, Microsoft is about to give Windows 10 PC owners their own free-to-play Halo game, sort of. The company will release Halo 5: Forge, the full map editor for Halo 5: Guardians, in the Windows Store on September 8. It will also allow those creators to play in online multiplayer matches with their friends.