Although OnePlus has yet to officially announce it, a new Open Beta build is now going out to the OnePlus 6 and OnePlus 6T. This arrives over-the-air for those who were already running the previous experimental build, as Open Beta 25 for the OnePlus 6 and Open Beta 17 for the OnePlus 6T.
As you can see from the screenshot to the left, the sole purpose of this beta release, aside from bringing some unnamed optimizations for system performance, is to prepare the two phones for "major upcoming updates". Clearly that refers to Android 10.
A few hours ago OnePlus outed the first beta builds based on Android 10 for the OnePlus 7 and OnePlus 7 Pro, and owners of the OnePlus 6 or 6T may have felt left out. But it looks like the company is actually working on switching its betas for the 2018 handsets to an Android 10 base as well.
In fact, these Open Beta 25/17 builds are very likely the last ones for the 6/6T based on Android 9 Pie, and we hope the Open Beta 26/18 builds based on Android 10 will hit yesteryear's flagships very soon.
Because OnePlus hasn't made the new builds official, you can't directly switch to them from the stable track. If you want to go the beta route, instead you'll have to download the previous build for your phone (OB 24 for the OP6, OB 16 for the OP6T) by following the instructions here, and then wait for the latest build to arrive as an over-the-air update.
Note though that while going from stable to Open Beta builds doesn't wipe your personal data, switching back to the stable track does.