As I noted in my review, there aren't a whole lot of differences between the OnePlus 6 and 6T, especially where software is concerned. Aside from a few small features the two ran nearly identical versions of Oxygen OS, all of which make upgrading from a OnePlus 6 a very hard sell. Now Nightscape mode, one of the major selling points of the 6T's camera, has come to the OnePlus 6, making the chance people will upgrade even less likely.
The OnePlus 6 did not have very good low-light camera performance, which didn't bode well for the 6T since it had the exact same hardware. The software end got a massive overhaul, though, improving night-time performance across the board. Nightscape Mode itself did a great job at reducing the amount of noise in low light, and now it's arrived on the 6T's predecessor as part of OxygenOS 9.0.2. We knew it was coming this was coming, but it's still good news for OnePlus 6 owners, or anyone who prefers a phone with a physical fingerprint scanner.
Other additions include the usual bug fixes and security updates, an overhauled designed for the 'About' phone page, a few new navigation gestures, optimised power consumption when the phone is in standby mode, and the option to activate an AI assistant by holding the power button for 0.5 seconds. OnePlus has also added 'Studio Lighting' to the camera to "improve face contour".
Interestingly there's more night camera news as well, since the Pixel 3's Night Site Mode has been ported to the OnePlus 6 and 6T as an unofficial app (via Ubergizmo). While not available in the Google Play store, they're the latest two phones to get a third-party version of GOogle's night-photography software. Seeing as how Night Site was a big selling point for the Pixel 3 you shouldn't expect anything official to arrive soon, but you can get the OnePlus 6 version on the XDA forums and the 6T's on Google Drive.
Usual warnings apply here, though, so install both of these at your own peril.