Halo tends to focus on its mainline series, but there have been some spin-offs throughout its history. One of these games was actually the cancelled project 'Haggar' - probably better remembered as a third-person-style Halo game set in the world of Mega Bloks.
Apparently there's been some progress since 343 Industries officially acknowledged footage of the Haggar build that appeared online in 2017. As highlighted by MintBlitz in a video on YouTube, an early prototype build of the game is now playable on PC using the emulator Xenia. This is all thanks to the efforts of a group of individuals known as 'Spark, Itchy.7 and Zoephie'.
This build is again a prototype including "a few missions" and there's even local co-op support included. The art design and "all major mechanics" are apparently close to done as well.
According to an official statement from 343's former studio head Bonnie Ross in 2017, Haggar "had a lot of fun ideas and invention behind it, but ultimately didn't progress beyond the early prototyping levels". 343 at the time also mentioned how this was just "one example of several similar projects" being evaluated.