How can I remove vibration when the phone is turned off on my Samsung Galaxy S9 (non-rooted Android 9, One UI 1.0)?
This is option is not for user’s control. It’s default for all smartphones.
The main purpose is to alert the user when their phone shuts down otherwise there is a chance that users may not know whether their phone is off or on.
So OEM provides this feature as default and cannot be controlled until the phone is rooted. Rooting a phone will give you a full control over the phone. But this is helpful. If you feel to remove this option, root your device but that is difficult fix to this. Mainly it is not a bug.
Hope it helps!