In the details provided in System Settings there were about 60 lines of info. There was/is one line: "For information on the security content of Apple software updates, please visit this website": https://support.apple.com/100100". This is a general statement so in plain English the whole thing is stating that there are no security updates in this one but when there is - then use the link. Elsewhere it's clearly indicated that 15.3.1 has "important security ...." so is it very poor communication or some coded way of indicating that the security updates are say only related to very few specialist users or some such.
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