Well this is going to create a huge stink... I can imagine many people leaving Github because of it. I mean if you're Amazon or Google do you really want your private software repositories in Microsoft's source control system? And if you're a hardcore open-source dev do you really want to be using any source control system controlled by Microsoft who waged war on open source and Linux for years?
Sounds like a great time for decentralized source control to become really decentralized...
All the details from Github here: https://blog.github.com/2018-06-04-github-microsoft/
and from Microsoft here: https://blogs.microsoft.com/blog/2018/06/04/microsoft-github-empowering-developers/