As spotted by Windows Central, Insider builds have removed an option to share your gameplay capture direct to Twitter and will direct you to share from your phone.
Players now have to send video clips to their phones or other devices if they want to share them on Twitter. An update to Xbox Insider builds also confirms that social media icons only show up in the "mobile sharing" option, emphasizing that a mobile device is required.
Since Microsoft hasn't officially made a response, it's still unknown why the change. It may not be ideal, though, if your phone is not with you when you're gaming.