A representative from the company took to the OnePlus forums to announce the end of the beta program for these devices:
To assure that our Open Beta program runs smoothly, and to prepare the addition of new devices to the program, OxygenOS Open Beta 6 will be the last open beta build for the OnePlus 6 and 6T.
The company recommends anyone currently running the beta software on a OnePlus 6 or 6T to roll back their devices. To make doing so easier, OnePlus is releasing a special rollback build that will install Oxygen OS 10.3.2 on user’s devices. Once that’s installed, device owners will continue to receive stable updates per the company’s normal release schedule.
Rolling back the device is a fairly painless process. Users will need to download the appropriate rollback package from OnePlus (this one for the OnePlus 6 and this one for the OnePlus 6T). From there, they will need to launch the File Manager and move the .zip file from the Downloads folder to the root directory. Next, open Settings, go to System, System updates, press the cogwheel on the top right, choose Local upgrade, and select the previously downloaded rollback file.
Unfortunately, rolling back the device and getting out of the Oxygen OS Open Beta will completely wipe all data, meaning that users will need to back up their devices in order to make sure they don’t lose their contacts, call history, messages, and so on.
Backing up a OnePlus 6 or 6T is fairly painless. Start in Settings, followed by Utilities, then OnePlus Switch, click Backup and Restore, and lastly select New Backup. Users will need to select the types of data they want to backup and then tap Start Backup. Once the backup is complete, it’s important to copy the data to a computer through a data cable or to a cloud service so it can be accessed after the rollback.
Once the device is completely rolled back, restoring the backed up data is pretty painless. First, copy the backup folder called [opbackup] back to the device from wherever it was stored. Like backing up the data, head to Settings, followed by Utilities, then OnePlus Switch. From there, tap Restore Backup and choose the data to be restored. This will get the device on a current build and out of the Oxygen OS Open Beta going forward.