There's a new 1904 Xbox Insider Preview on the way to the Preview Alpha ring today with the build string 19h1_release_xbox_dev_1904.190122-1520. It contains the second new feature to be announced for the 19H1 development branch, with the first being the new Mini Keyboard.
Now, users will be able to reprogram buttons on the Xbox Media Remote. Interestingly, this will start with the button to access OneGuide, so you can now use that button to launch a different app.
If you're unfamiliar with OneGuide, that's OK since it doesn't have as much of a purpose as it used to. OneGuide was made for one of the original Xbox One's defining features, which was HDMI passthrough. You could hook up your console to your cable box, and OneGuide would show you what's on. It mostly worked with Kinect, so you'd be able to say things like, "Xbox, watch HBO". But of course, Kinect has been killed off since then.
Of course, the change is also good for cord-cutters. Users can now use that button to access their favorite streaming app instead of OneGuide.
While the release notes specifically called out the OneGuide button, Microsoft did imply that other buttons will be programmable as well.