why has my settings icon disapeared

Use App Library to search the Settings app, see -> Organize the Home Screen and App Library on your iPhone

Organize the Home Screen and App Library on your iPhone - Apple Support           

• If you have find the app, tap & hold on the app, then drag to the Home screen & drop it.

Bonging Said:

"Why has my settings icon disappeared"

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Troubleshooting Disappearing Widgets:

A. Drag it Back:

Swipe your finger in from the right. Then, drag Settings back over.

B. Why it Disappeared:

As to why it was disappeared, is anyone's guess. If certain wouldn't on its own, unless the GUI was changed by Apple.