Stop iOS auto-downloading updates
If you've already turned off Updates in iTunes &amp; App Store and deleted the update in Manage Storage, then your final option is to totally block the update domains


https://appldnld.apple.com
https://mesu.apple.com/


in your hotspot.  This will block all software updates to any device connected to the hotspot.  [1]



However, a variety of reliable sources (NY Times, USA Today, WSJ, Associated Press) would recommend updating to iOS 12.  Many of them cite speeding up older devices as the primary reason.