Hello nic_cyr,
Thank you for using Apple Support Communities!
I understand from your post that you recently updated your iPhone 8 and have not been able to find the iTunes Extras feature.
First, make sure your iPhone is in landscape mode while playing the movie. Once in landscape you should see the Extras button along the control bar, as shown in the image below: Watch shows and movies in the Apple TV app on iPhone
This article will provide you with steps on how to view your iTunes Extras if the above doesn't work:
If you can't view your iTunes Extras - Apple Support
Best Regards.