Is there any howto or link on the procedure? Google didn’t help much.
Given that different router has different ways to configure this, Google will have a hard time giving a comprehensive answer. The basic idea is finding out features similar to blacklisting certain domains such as some form of parent control or firewall stuff. I would go and read the router’s manual before configuring it. Many cheap routers may not provide such functionality.
Another geeky way to do so is installing a Raspberry Pi and configure Pi Hole as your home hardware ad blocker (or even firewall). It runs Linux, and can easily deal with banning iOS update at your home Wifi network.
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But this would only work at home. As soon as you connect to another wi-fi it will download again.
Yes, you are correct. But, before Apple gives user this feature (like the one in iOS 13.6), that is probably the best we can do to block it as much as possible.