How to prevent iPad and iPhone from downloading and updating iOS?

Question

iOS

(This is necessary to prevent unauthorized updates of the operating system by users).
As an option - try to install a profile created using Apple Configurator 2 or Profile Manager in the device, specify the IP addresses or domains of Apple update servers in the Content Filter section of the profile ...
Disadvantages of this method:
- only suitable for "managed" (supervised) ) devices;
- Apple update server addresses are not particularly public, and it is not a fact that they will not change in the future;
- this restriction seems to work only for browsers :(
(It seems to be possible to prevent iOS updates (again, only for managed devices) using an expensive and cool MDM system, but the budget does not allow such chic).
Suddenly someone knows a working convenient option for solving this problem?