With the new software support lifecycle, OnePlus announced that they'll soon stop releasing new Open Beta versions of OxygenOS for OnePlus 3 and OnePlus 3T.
This means, that you'll no longer get any beta updates from OnePlus. Hoewever, they do offer a conversion update to switch back to the regular updates (which will soon be security updates only, but that's a different story).
What does this mean for me?
This change does not affect the regular functioning of your device. No files or settings should be erased when the conversion update gets installed. The only changes you need to make are within Oxygen Updater itself.
At first, Oxygen Updater will offer you the final beta version and the conversion update. Hoewever, after you've installed the conversion update, you'll have to switch back to the regular updates. To do so, tap the three dots in the top right, then tap on Settings and select either Full Update or Incremenetal Update next to "Update Method".
Finally, please note that Open Betas will still get released for OnePlus 5 and newer devices. If you're not using Open Betas or have a OnePlus 5 or newer, nothing will change for you.