According to the information at hand, the Z390 still continues the LGA1151 V2 interface, which is backward compatible with the 8th generation Core CPU. However, like the H370/B360, the Z390 has added the native Gigabit Wi-Fi and USB 3.1 Gen 2, but the lithography process still stays. At 22nm.
As for the 9th generation Core processor, the latest statement is announced on August 14th, and the paper will be released in October. It will be listed at the end of the year and early next year. At present, including Asus, MSI has updated the BIOS for its 300 series motherboards to support Core 9000.