How to recover accidentally deleted Flashlight on iPhone 15

To add the flashlight back to your Control Center: Open the Control Center, tap the + in the top left, in the bottom tap Add a Control, add the Flashlight back, then tap any empty space to save.

To add it back to your Lock Screen controls: Hold on the Lock Screen, tap Customize in the bottom, tap on one of the two icons in the bottom, search for Flashlight and add it back, tap Done in the right corner.

And where are you looking in Settings? The only place I know where it's in settings is for the Action Button, and I've never seen that disappear before.

You're welcome, glad it helped. Have a great weekend.