The 9th Gen Core series have appeared in official documents.
Intel Coffee Lake S enters 9th generation
The 9th Gen Core series are now confirmed to be utilizing Coffee Lake architecture, as revealed by microcode update guidance file. The Coffee Lake S series featuring 6+2 and 4+2 configurations are now listed with Core i5-9600(K), Core i5-9500(T), Core i5-9400, Core i3-9100 and Core i3-9000 SKUs.
This ultimately confirms that 9th Gen core series will be using Coffee Lake S, not even a different codename as previously thought.
The file also indicates that Core i7 series will likely utilize 8-core configuration since such configuration is missing from the document.