Many cell phone repair businesses are starting to see a rise in cell phone battery issues such as iPhones not holding a charge.
New operating systems often use more power than previous operating systems. It is common for battery life to drop on older devices that have upgraded operating systems. iOS 11 was designed for the iPhone 8 and X, and has been problematic since its release. Recently, Apple has released 2 more updates which has improved battery time by 159 minutes.
If the battery drain is still too intolerable, you can always switch back to iOS 10.3.3, though it is not advised.