Most phone manufacturers have integrated the Google Discover feed into their home screen launchers for years, but OnePlus only started doing it with the OnePlus 8 series. The Discover feed completely replaced the old 'Shelf' pane that was present on previous models, but OnePlus later brought it back with a settings option, and now the company is testing an easier toggle for the process.
OnePlus 8 owners have already been able to switch between the old Shelf and the Google Discovery feed by opening the launcher settings (hold down on the home screen and tap 'Home Settings'), but now OnePlus is experimenting with quick access toggle on the top-right of the Shelf/Discover page. 9to5Google published a video of the feature in action, seen below.
However, the switch seems to be a server-side test. Even if you sideload the required update, it's only working on some OnePlus 8 devices. Unsurprisingly, the feature also doesn't seem to show up on devices that never had the Discover feed to begin with (like the unlocked variants of the 6T and 7 Pro).