GitHub Next is a branch of GitHub that “investigates the future of software development.” From the website:
We are a team of researchers and engineers at GitHub, exploring things beyond the adjacent possible. We prototype tools and technologies that will change our craft. We identify new approaches to building healthy, productive software engineering teams.
While the GitHub homepage recently switched to the custom Mona Sans, GitHub Next still uses Alliance No.1. Both Alliance and Mona Sans – which appears to be an evolution of the latter – were designed by Degarism Studio.