Here is your solution.
I too have a limit on my data and their updates aren't exactly small in size but are currently about 2.5 GB in file size which is quite expensive if you are on restricted data.
For years I had to manually keep on deleting these updates so that I didn't accidentally install the update and to get rid of those relentless notifications, but good news I have finally a solution that works for me.
What you need to do essentially is fill up the space on your device enough so that the IOS Update physically cannot download. It will present you with a message stating that "The update cannot be installed because it requires at least #### amount of availiable storage during installation."
This will then solve your problems.
Your choice as to how you fill up the space. You may want to download a lot of Netflix movies to your device.
For me the quickest way may seem obsurd but it's the quickest.
I set my iPad facing a window and simply just hit record and leave it for an hour or so. At least I get some nice wildlife shots as my window sits facing a park. This usually fills up the space and if I ever need space I simply go in to my gallery and delete the video. You can even do incremental videos so you only have to delete one for however much space you need rather than having a single file taking up the entire space on the device hard drive.
This may seem tedious but is the only way to get Apple of your back from forcing their stupid updates on its users.