A OnePlus employee has officially confirmed that the company no longer supports the OnePlus 6 and 6T. Both devices received the November 2021 security patch as their last update, along with three major Android upgrades before that.
https://www.notebookcheck.net/The-OnePlus-6-and-OnePlus-6T-will-no-longer-receive-official-software-updates-from-OnePlus.593357.0.html