Best answer: The OnePlus 6T's in-display fingerprint sensor will work with a screen protector, but OnePlus isn't saying that it will work with every screen protector you can find. Only screen protectors from OnePlus and its partners are guaranteed to work, so it's best to stick with one that comes directly from OnePlus, at least for now.
This is a whole new world of fingerprint sensors
In-display fingerprint sensors are the way of the future, and they're slowly taking place of the capacitive sensors we all know and love. These new in-display units are beneficial because they don't take up as much room inside the phone, and don't require the rear design of the phone to be changed in order to accommodate the sensor. One of the downsides, though, is that they're more finicky about what goes on above the sensor — the display panel and glass itself, to start, but also anything you put on top.
The fingerprint sensor is designed to work with a screen protector — but it's not a guarantee with every one you find.
These in-display sensors work like a camera, with an actual image sensor that uses the light from the display panel to illuminate your fingerprint and capture an image that's compared to what was initially scanned. By adding another layer to the equation, like a plastic or glass screen protector, it can affect the way the sensor "sees" your fingerprint — just like putting more layers of material in front of your camera lens.
The OnePlus 6T comes with a pre-installed plastic screen protector out of the box, so we know that the fingerprint sensor will work with this extra layer of plastic. What OnePlus can't guarantee, though, is that any random screen protector you buy online will work the same way. Right now OnePlus is saying that you can only use screen protectors that it sells directly, or from yet-to-be-named partners that will certify that their protectors work with the new sensor.
Thankfully OnePlus is quick to offer its own screen protectors, with a quality
OnePlus 6T 3D Tempered Glass Screen Protector
For the moment, the safest way to go is to buy a screen protector directly from OnePlus.
This is a new world where screen protectors influence how well your fingerprint sensor works. If you like to use a screen protector, start with the ones that OnePlus sells directly for the 6T. If you want to use another brand's protector, make sure that it certifies that it will not interfere with the 6T's in-display fingerprint sensor.