Persistent update notification on iPhone with automatic updates off

Did you install the latest update? Good idea to do so considering the security updates in the iOS. I suspect given the text of the message you describe, the iOS update has downloaded to the device and is waiting to be updated. It won't automatically update, but it can automatically download after a period of time.

You used to be able to go into Settings and delete the update, however it will keep prompting you about the update for a while until you install it. I don't remember the entire method for doing it, but if you search here, you can probably find an older post about it. Battery issues after iOS updates is something that will occur, but generally settle down in a couple of days after the phone has had a chance to finish all of the tasks necessary for the update, including indexing files again.