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
This document lists security updates and Rapid Security Responses for Apple software.support.apple.com