I don't think there's a definitive answer necessarily, but security-wise, I would expect it to be supported.
On the one hand, there should be an hardware compatibility issues between Skylake-X and W7 or W8 as far as architecture - and with extended W7 support ongoing, it *should* be supported.
On the other hand Kaby Lake doesn't officially 'support' Windows 7. While it runs in most cases, there's no guarantee of driver support etc.
From a security standpoint, I don't see why it wouldn't be 'supported in that sense'. They're patching the OS, and providing the hardware is capable of running the OS (which it should be, albeit potentially lacking driver support), then the OS will be patched.
With no guarantees of driver support even short-term (let alone longer term), I wouldn't think W7 + Skylake-X would be particularly great idea, but from a security standpoint should be safe.
*EDIT*
Older articles stated:
http://www.makeuseof.com/tag/windows-7-wont-work-intels-current-next-gen-cpus/
Microsoft is discontinuing support for these versions of Windows on Skylake on July 17, 2017.
After that point, only the “most critical” security fixes will be released for those platforms. There’s no guarantee of that either, as these fixes will only be released if they don’t “risk the reliability or compatibility” of Windows 7 and 8.1 on non-Skylake systems.
Then it was pushed to 2018:
http://www.zdnet.com/article/microsoft-pushes-back-windows-7-on-skylake-support-cut-off-date-from-2017-to-2018/
Now the official line is only 'supported devices' will get full support (OEMs generally)
http://www.zdnet.com/article/microsoft-extends-again-support-for-windows-7-8-1-skylake-based-devices/
List of supported devices:
https://support.microsoft.com/en-ca/help/11675/windows-7-windows-8-1-skylake-systems-supported
Now I read that to mean security patches will be there for all, but it's pretty ambiguous.