The upgrades are not mandatory, but the downloads certainly are. I've had to delete the iOS downloads several times over years (including the downloads for iOS 12) because iOS doesn't have an option to disable this, and it automatically downloads the upgrade in the background when on WiFi for a long enough time. It's a cumbersome process to delete the download. It's even worse when you consider that a downloaded update would be installed overnight if you leave the device on the charger and connected to WiFi and by mistake tap on install when that prompt appears at some random time. The user interface for controlling and managing iOS upgrades does not exist.

> This is how you get phones running out of battery halfway through an update.

The OS could always put a limit saying the phone should be charged at least 30-40% for an update to start installing. Even now, the upgrades and updates are allowed to go through for iPhones without them having to be on the charger.