Based on what researchers who had access to the Windows kernel are saying, Windows has a decent amount of edge cases and special conditions to maintain backwards compatability with old software. For example if some popular software from 2000 abused some implementation detail that has since changed, Windows will detect that specific program and simulate old behaviour for it.

I expect most of that stuff lives in the userland interfaces to the kernel, but at least Microsoft seems to consider those to still be kernel code.