So yes, think it is safe to say that Z390 is a real thing judging from the info that has surfaced the past few weeks. New in the line of anything with photos, ASRock, ASUS and MSI Z390 motherboards. These have surfaced through the typical channels.
Earlier this week the line-up from ASRock leaked, no no photos can stay hidden it seems, most other manufacturers board photos have leaked likely through distributor websites. There is a pretty extensive number of photos available from VideoCardz with Z390 motherboards; this time from Asus, MSI and ASrock. It practically is the entire line-up, which you see in the pictures below. Z390 would be a revamped Z370 chipset, optimized for the new Core 9000 series processors (which will also work on Z370 by the way).
Have a peek.