Using a manual camera app like Halide an iPhone 6 is able to achieve a maximum of 1/2" exposure time, what's the longest an iPhone X can reach?
It is dependent on the Third party app you are using and not the phone itself.
I have several apps that allow me to use shutter speeds from fractions of a second up to 60 seconds and even bulb ( as long as you want ). I have used these apps on my iPhone 6 and my new iPhone 8+.
I use Procam 5 as my main app but i have others. It has a slow shutter mode. ( i am not affiliated with procam, just a user )
Do some research when you are choosing an app to see if you can find out what the manual control options for each app.
EDIT: I asked the pro cam five support staff the question and here is there reply
"Hi , Thanks for getting in touch with us. The longest exposure time supported by the iPhone X hardware is 1/4 sec. Longer exposure time are digitally simulated. Please don’t hesitate to contact us again with additional questions or feedback. Best",
Not sure if these are the limits of the camera assembly, or the operating system, or the application I am using, but using Halide I can set the shutter speed between 1/3"-1/50k" on my iPhone X.