Motherboard makers are starting to roll out BIOS updates to add support for the upcoming Intel Core i9-9900KS processor. So far, ASUS and ASRock have issued updates for some of their Z390 motherboards.
Tom's Hardware compiled a list and speculates that not all boards will support the Core i9-9900KS. The chip's TDP is still unknown, but it's pretty much guaranteed to push beyond the 95W TDP of the 9900K, so that may result in lower-end motherboards not being able to push enough juice. You will still be able to use the chip on these motherboards, but you'll be hit by performance throttling.
On ASRock's side, the manufacturer has rolled out updates for the Z390 Phantom Gaming X, Z390 Taichi, Z390 Steel Legend and Z390M Pro4 motherboards. As for Asus, the brand has started to update the Z390-based ROG (Republic of Gamers) lineup, such as the Maximus XI Apex, Maximus XI Gene, Maximus XI Hero, Maximus XI Hero (Wi-Fi) and Strix Z390-E Gaming motherboards. First priority is likely going to the better sellers; however, it's likely that not all Intel 300-series motherboards will support the Core i9-9900KS either.
The Intel Core i9-9900KS is expected to hit the market in October. This eight-core model promises a 4GHz base clockspeed and a 5GHz Boost on all eight cores.