George has an iPhone 7 and he keeps getting asked to update iOS 11. His wife doesn't want to upgrade due to battery life issues. Leo says that he has iOS 11 and it runs just fine. A certain percentage may run into problems, even 1/10th of 1% would still be thousands. That's just the nature of the business. George doesn't know how to stop the notifications of iOS 11 other than to just ignore it. Once he upgrades, he can't go back. That's why the conventional wisdom is to wait a few weeks on updates, because there will be patches coming out in short order.
From the Chatroom - Go to Settings > General > Storage > Manage Storage. He's getting the notification because the phone has already downloaded the update. If he deletes the update, the notifications will stop until it downloads again.
He also doesn't like being bugged by Facebook. Leo says he can turn off notifications in Facebook's user settings under notifications. There's email, text and popup notifications. He can just turn them all off. He should go to Facebook on his terms, not theirs.