ImmersiveHost is an attempt to host Immersive mode applications ("Windows Store applications", "Metro applications", "Modern UI applications", "UWP applications", etc.) in a more open context outside of the usual app container/package situations.
This was an experiment around March of 2016, and was developed referencing mainly TH2 (10586/1511) and then-contemporary RS1/1607 releases of Windows 10. It also specifically targeted only a small set of applications, and probably would take some effort to get working again.
Intent to continue does exist, but is kind of a low priority for the time being, so this is mainly useful as a case-study into the Immersive application framework on Windows, of which remarkably little documentation exists.
vendor/ folder is missing as it kind of used broken dependency management and didn't really track commit hashes - a list of vendor trees is shown below: