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.
"With continued, great feedback from the community and after careful consideration, we have decided to remove Mixer from Xbox Game Bar to focus on delivering new, more often requested experiences across Mixer, Windows 10 and the Xbox Game Bar. You can broadcast to Mixer from Windows 10 devices using third-party tools such as OBS, Streamlabs, and XSplit and continue to use Xbox Game Bar for screen capture and sharing, controlling your music and audio settings, finding new teammates with Looking for Group (LFG), and chatting with Xbox friends across Xbox console, mobile, and PC – all without leaving your game."
Microsoft also quietly pruned the Xbox Tournament features from the OS recently, as well, perhaps as part of a wider effort to cull underused gaming features across the board. Either way, XSplit or OBS provides a far more feature-filled streaming experience than the Xbox Game Bar ever did for Mixer, so it's unlikely very many people were actively using it.