It would be nice if Apple would explain how this will help anybody apart from themselves. I understand that it is supposed to stop jailbreaking, and therefore allowing those with bad intentions getting where they didn’t ought to be, but I’d like some words. I still haven’t loaded the last one !
It's all here: https://support.apple.com/en-us/HT210393
Apple won't go into too many more details about security vulnerabilities as they don't want people like you who are on older versions any more vulnerable than you already are. Patching security vulnerabilities helps everybody using the phone, not just Apple.