Unfortunately this is no longer true. If you delete it the device just downloads it again. Turning off auto update in settings does nothing to stop it, either. Apple's Twitter support account confirmed this for me:
"@darc_y Once your iPhone is up to date, you'll no longer receive those prompts."
"@darc_y We understand. The alert you're receiving will continue to appear as long as there is an update available on your device."
For better or for worse, Apple wants everyone on the same iOS. And macOS -- my 2011 mbp 15" gets regular prompts to update to Sierra and my only options are now or later. Sad.