Older os support is generally related to if drivers for that OS are still available. No driver, no support. And a lot of times the manufacturer will have support while a mobo support page won't have it listed. It was the same with xp and vista.
For the most part, even if Intel does not officially support an OS or Microsoft does not officially support a given CPU on a given OS version, the generic Microsoft drivers will still be enough get you up and running anyway, you just won't have performance and functionality beyond what the generic drivers support.