Mixer goes byebye with the most recent update to the Xbox Game Bar.
When Microsoft originally purchased Mixer (then, Beam), it very quickly integrated the streaming service directly into Windows 10, allowing users to begin streaming at the touch of a button (or, well, a key combo). Moving forward, that will no longer be the case.
Microsoft began working on a sweeping revamp of its Game Bar features for Windows 10, spinning it out to a separate app, and iterating updates based on user feedback. With the most recent update, Microsoft added an FPS counter to games and an achievement overlay. However, it also removed a fairly major feature.
Full story from the WindowsCentral blog...