I suspect they will patch serious security issues if they need to. Its tough to say though. They've been doing N-1 with iOS even though they do N-2 with Mac OS.iPhone 6s (all models), iPhone 7 (all models), iPhone SE (1st generation), iPad Air 2, iPad mini (4th generation), and iPod touch (7th generation)
Apple security releases - Apple Support
This document lists security updates and Rapid Security Responses for Apple software.support.apple.com