I assume that the reason the Skylake processors were still released in late 2017 was because they are still supported by Windows 7/8? This is despite Kaby Lake/Coffee lake already being out.
While support for Windows 7/8.1/10 is provided for Skylake processors, support for only Windows 10 is provided for the Skylake-X and Kaby Lake-X processors that you can use in your X299-based board. This is because Skylake-X was released after the deadline Intel established for their transition to supporting only Windows 10.
Hope this helps,