In 2015, Microsoft partnered with 343 Industries and Sabre Interactive to create Halo Online exclusively for the Russian market. Due to some game development issues, Halo Online was put on hold indefinitely, much to the disappointment of Halo fans. The project was taken up independently, however, using some apparent reverse engineering, and a group known as ELDorito released their own version of the online multi-player game. According to its website, ELDewrito is a "free community-driven Halo experience."
Since ElDewrito is an open-source mod and not a Microsoft asset, it will not be taken offline. However, since its 0.6 release, Eldewrito uses some Microsoft assets required to play the game (MS23 builds, Full 0.6 game zips, etc.), so those game aspects will be removed by Microsoft.
In a separate statement on ElDewrito's website, the team announced that they will have to "temporarily halt development until more information is available, and we must honor this request. During this time, please do not make threats or harass 343 Industries." ElDewrito's blog post was made in response to an official statement posted on Halo Waypoint, some of which can be found below.
"Today, we want to let our community know that Microsoft has initiated actions to protect its Halo intellectual property in the wake of the recent “ElDewrito” PC release. Community created content has long been a key pillar in the Halo franchise and something we have continuously sought to support – from the early beginnings of Red vs. Blue to Forge-made maps & modes to the “Halo Custom Edition” to original recent fan creations like “Installation 01.” These projects, and others like them, have one key factor in common – they fit within Microsoft’s established content usage guidelines.
We’d like to clear up a few areas of confusion we’ve seen across the community over the past few days regarding ElDewrito and other fan-made projects. In the case of the original Halo Custom Edition, that was as a specific add-on to Halo PC to officially empower the mod and content creation community to essentially go nuts with Halo: Combat Evolved (it even required a valid Halo PC retail key). More recently, Installation 01 has garnered some buzz and even made headlines for receiving a thumbs-up from 343.
Installation 01 is an original work, built from the ground up in a separate engine, that abides by Microsoft’s content usage guidelines. With Halo Online, there’s a common misconception that once it was canceled, the assets were either turned over as “open source” or left for the community’s whims as “abandonware” – neither of which is actually true. Not only did Microsoft issue takedown notices at the time of the original leaks, but many elements of that underlying code and content are still actively being used today and will continue to be in the future."
So for the time being, there will be no further game development on Halo Online until Microsoft and ElDewrito figure this out.