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.
Mixer broadcasting direct from Windows 10 via the Game Bar is no longer supported. Instead, Microsoft advises users to opt for a tool like OBS, StreamLabs OBS, or XSplit, which the vast majority of Mixer streamers probably do already anyway.
A Microsoft spokesperson provided this comment to Windows Central on the matter.