OnePlus announced in a forum post that the software support for OnePlus 6 and 6T has ended after 3 major updates and more than 3 years of updates.
The last major update of the two devices was OxygenOS 11 based on Android 11 late last year. Following this, there was an Android security update in December 2021.
This isn’t too surprising since OnePlus 6 was unveiled in May 2018 and OnePlus 6T launched in October 2018. Keep in mind that you can still use either device still, they just won’t have any more updates coming. Which might be a good sign to prepare to upgrade.