I received an EVGA Z370 FTW motherboard direct from EVGA in mid-September 2018.  The board shipped only a few weeks after the 1.09 bios added "new Coffee Lake-S Processors" (the 9th gen CPUs) but I suspected my board didn't have that update on it.  I was finally getting around to building a new rig but realized the CPU shortages and 8th gen parts having a price premium over 9th gen was likely to leave me in a squeeze.
 
I did some more research and noticed that intel's microcode file for the 8th gen Pentium Gold/Celeron parts (G4900-G5600) is in the same CPUID as the i3-8xxxx models that the z370FTW does officially support - 906EB.  I didn't want to involve an 8100-level part just for bios flashing, but did eventually find a $45 G4920 CPU.  Tonight I used that to flash my board's bios from 1.08 to 1.12 without a hitch.  I can't speak for all the z370/z390 boards, but I suspect the CPU comparability charts bottoming out at the i3-8100 isn't the cheapest CPU you can use, thanks to having a shared microcode file with the Celeron/Pentium Gold CPUs.
 
It's time to leave my 2500k system behind, although I might have to hold onto this bios-flashing CPU until EVGA releases a bios that supports the 9th gen R0 stepping just to be safe.  I'm thinking 9600k for this build.