Many Instagram users were surprised today when their Explore feeds were scrolling in a new horizontal layout instead of vertically without any prior warning. As it turns out, this was not intentional.
A test of a new layout, which utilized a lot of the design language of Instagram's relatively recent Stories feature, was only meant to be just that: A test that wasn't for any sort of public distribution. However, somewhere along the line, this test was rolled out to several users.
As of now, no formal announcement has been made about plans concerning the formal rollout of the new Explore feed layout by Instagram or its staff.