On the OnePlus 6 and 6T's community forum, a spokesperson announced that the company will no longer be providing software support for the devices that were released in 2018.
A three-year software support limit for the company's OnePlus 3/3T smartphones isn't exactly a surprise. There are three major OS updates and security updates every OnePlus flagship phone, and the OnePlus 6/6T ran out of time in 2021. OxygenOS 11 for Android 11 was last updated in September of last year, and it received the December security patch just recently.
To keep using your phone after the OnePlus 6/6t is out of the question, there are third-party ROM options or accepting that you won't be able to acquire the latest Android features.