In Short

To bring the Windows and Xbox feedback programs closer together, Microsoft closed down the Xbox Preview Program in mid-2016 in favor of the all-new Xbox Insider Program.

Xbox Insider Program Changelog

The new Insider Hub brings together all feedback and app beta tests into a single place, allowing Xbox console owners to tailor what updates they opt-in to. Everyone who was in the previous Xbox Preview Program is eligible to join simply by updating their Xbox Preview Dashboard. Microsoft isn't accepting new testers at this time, but we'll be sure to let you know when they begin.