It has something to do with ransomware because you were the one to brought the idea about open OS and ransomware.
The 3rd link which was written 1979 applies to any modern OS. All modern OS protects the user through UI and abstraction from the intricacies of the OS. If I were too use the term open and closed system in context with iOS and Android OS it has something to do with locking down the OS. In Android the OS does not lock down the system to a point not limiting system customization and tinkering of the OS itself as opposed to iOS where you users have very limited option when it comes to customization not just in UI but in the whole system. Like in iOS jailbreaking is not supported but in Android rooting is tolerated and even installing different ROM is allowed.
The fact is your term of "open operating system" is rarely used in todays information tech era. Even Wikipedia does not have any information about your idea of "open operating system" because I think it does not describe any modern OS. Modern OS are open in a sense that they allow developers to work with it. Some of course have limited API and features like iOS but it does not mean they are closed OS, I think the best way to describe them is they are more of a lockdown OS but not a closed OS. Perhaps some embedded OS can be called a closed OS because once they are deployed there is nothing you do with it. But that would be a rare OS since most of the embedded OS are running Linux kernel which is an open source os and is very hackable in a sense you can do whatever you want to do with its kernel.
About your last paragraph, what I can say is "close" or "open" OS it matters very little when it comes to ransomware because this kind of malware thrives on exploiting bugs or features of the OS. It has little to do with being "close" or "open" but more on how robust and bug free the system is. Well if you are running Symbian OS for your keypad phone which has very little to none when it comes to installing 3rd party apps then I guess you could say its malware or ransomware free because afterall ransomware starts with social engineering. They don't install themselves to the system. Anyway I think we are way out of topic already

I rest my case.