[xxs]

>many existing motherboards did not have enough power delivery capability to support the planned 6 and 8 core part

Coffee lake's 8700k had 6 core not 8, the refresh lake-s did feature 8 cores. The release TDP of 8700k was the same as 6700k - 95W, of course it consumed more at peak, and when overclocked.

The support would still depend on the motherboard manufacturers adding the CPU support (along w/ the microcode) to their BIOS. Many high-end boards, e.g asrock z170 extreme6/7, would have supported 8700k.

The situation is no different today that many (most) boards do not top support top end processors anyways, due to poor VRMs, even when the socket is the same. (or if they support the CPUs are effectively power throttled)