Looks like you may not need a Z390 motherboard for Intel's eight-core chips after all. We wrote about a MSI BIOS update a couple of days ago and now more beta BIOS releases are popping up. Various Z370 motherboards from ASUS, ASRock, and MSI now have beta BIOS versions with the Intel 06EC microcode revision.
Intel's changelog reveals the 06EC microcode update includes mitigation against newer Spectre variants, and an older revision of the document also mentioned support for Intel Core 9000 series processors. These new processors are expected later this summer.