Description:
1.
I tested samples of A-Frame.
Here is a video with iPhone X on A-Frame samples.

And error messages in the console.

2.
I also tested on webVRPollyfill demo

It has no error message.
3.
I tested various version of A-Frame on glitch sample.
Sadly, it doesnt work in 0.9.2, 0.9.0, 0.8.2 and 0.8.0.
not only iPhone X, but other devices also don't work.
Only Pixel2 works well.
Unfortunately, Chrome and Safari now ship with devicemotion events disabled by default. The user has to enable manually on settings:
Content also must be served over https
Additional discussion https://github.com/aframevr/aframe/issues/3976
There is no longer a "Motion & Orientation Access in Safari iOS" option any more in iOS 13 Beta 3
As described in the referenced issue:
iOS 13 has a requestPermission API
Most helpful comment
Unfortunately, Chrome and Safari now ship with devicemotion events disabled by default. The user has to enable manually on settings:
Content also must be served over https
Additional discussion https://github.com/aframevr/aframe/issues/3976