Microsoft Extends Windows 7 and 8.1 Intel Skylake Support

User space remains compatible, although sometimes new instructions are added that would cause a fault if run on an older processor. That is a known and understood issue with things like cpuid and compiler flags being able to choose what to do.

Privileged (kernel mode) can change. It is roughly backwards compatible, but sometimes there are changes that when taken into account result in a more optimal system. For example power management keeps getting more and more complex as more processor components are affected, there are interactions and latencies to be understood etc. If you ran older naive kernel code, it could cause considerably higher temperatures and power consumption, possibly even leading to throttling. It may make poor scheduling decisions leading to lower performance. It won't take advantage of newer protection and performance mechanisms. Worst case it may not do context switches properly leading to bizarre crashes or information leakage.

From the actual blog @ https://blogs.windows.com/business/2016/08/11/updates-to-sil...

"Windows 7 was designed nearly 10 years ago before any x86/x64 SOCs existed. For Windows 7 to run on any modern silicon, device drivers and firmware need to emulate Windows 7’s expectations for interrupt processing, bus support, and power states- which is challenging for WiFi, graphics, security, and more. As partners make customizations to legacy device drivers, services, and firmware settings, customers are likely to see regressions with Windows 7 ongoing servicing."

Except this is a reality that was blindingly obvious to people outside of Microsoft, and yet they maintained their ridiculous stance for many months.