Blocking iOS Wi-Fi automatic update downloads?

Without jailbreaking, or using an MDM (mobile device management) solution, you could just block the Apple site at your router. The site URL is appldnld.apple.com. All of Apple's IPSW files come from that location.

The latest update to the evasi0n jailbreak, evasi0n 1.2, allows you to disable OTA updates.

This is a fairly good tailored version of evasi0n considering the fact that it's the second update it received since it was released if you are hesitating about the down sides of jailbreaking on device performance.

Turning off iCloud might be a disadvantage. If you really want to disable OTA updates; I strongly suggest you to jailbreak.

Here is a tutorial for jailbreaking with evaisi0n.

P.S. If you will download evais0n, ONLY download it from evasi0n.com. Do NOT trust other sources.

Blocking iOS Wi-Fi automatic update downloads?

iOS/iPadOS 13.6 has introduced the ability for users to turn off automatic download of updates. To configure automatic download or installation of iOS/iPadOS updates on your iPhone or iPad, go to Settings app → General → Software Update → Customize Automatic Updates and toggle the switches as desired.

The linked Apple Support document discusses the same:

• https://support.apple.com/kb/HT204204

Customize Automatic Updates

With iOS 13.6, or iPadOS, you can choose not to download software updates automatically. Go to Settings > General > Software Update > Customize Automatic Updates, then turn off Download iOS updates.