OnePlus has just announced the end of the Open Beta program for its 2018 flagships, the OnePlus 6 and 6T. Open Beta 6, which rolled out earlier this month, will be the last update that the phones see in the company's beta program. A new "rollback" build is available to migrate back to the stable release, but take note: It will wipe your device.
This doesn't mean the end of updates for either phone. OnePlus promised a minimum of three years of software updates for all its devices: Two of "regular" Android version updates, and one of security-only updates, and we have plenty of time before we hit that point. While the OnePlus 6's two-year anniversary is coming up next month, the 6T's isn't until November. Generally speaking, the phones share software builds, so we can probably expect to see updates until at least fall 2021.
Those of you running the Open Betas on the OnePlus 6 or 6T will probably want to migrate back to stable soon, as it will be the only way to get updates going forward. Full details on how to perform the migration have been published to OnePlus' forums, but the short version is that you should download the required rollback package for your phone model, drop it in your phone's root storage directory, and flash it manually with the option built into the system updates pane. You should also probably back up your device, since this will wipe it. OnePlus recommends using its own OnePlus Switch utility to do that, though you'll need to remember to copy that backup off your device somehow (like via USB OTG, network storage, or MTP if you have to).
For more details and links to the rollback packages, you can read OnePlus' full announcement and instructions below.