A Mercurial compatible SCM (not sure if it is a fork) built for their workflow (monorepo) and scale (enormous, git is not usable at their scale, at least for a monorepo). Uses Python and Rust. Designed for efficient centralization rather than decentralization.
I thought Microsoft was working on a "mod" to git that made it work on huge repos, e.g., the Windows source. Did that ever come to fruition?
First it was https://github.com/microsoft/VFSForGit then replaced with https://github.com/microsoft/scalar and then into their git fork https://github.com/microsoft/git