AMD has finally released a slide that outlines CPU support for various motherboard chipsets. Backwards compatibility is hampered due the unreasonably small 128 megabit BIOS chip used motherboards prior of X570 which uses a 256 megabit BIOS. Part of the problem is the cost of the 256 megabit hardware is more than double the cost of the 128 gigabit part.
AMD is only supporting Ryzen 3000 series processors on the new B550 motherboards. MSI introduced the MAX series motherboards with B450 and X470 redesigned with a 256 megabit BIOS chip. Probably will not be long before 512 megabit variants emerge unless some change is made.
My R5 2400G does work on the MSI X570-A PRO as of AGESA 188.8.131.52 but the performance is not nearly as good as the R5 3600 on the same motherboard. Several new BIOS releases for the MSI X570-A PRO have improved the situation considerably.
I have other AM4 motherboards where the R5 2400G can be used. The X470 Gaming Plus and the B350M Bazooka are available. Another chassis and PSU could being another machine into service.
Found this on twitter showing a R3 1200 which is the cheapest CPU on one of the most expensive X570 motherboards. This is backwards compatibility that is not impossible
The R5 2400G is convenient as it runs on A320, B350, X370, B450 and X470 platforms. This makes the CPU useful for BIOS updates as many motherboards do not have the Flash BIOS feature of the MSI X570-A PRO.
AMD has bumped up the infinity fabric timing so that faster variants of DDR4 will work. The G.Skill Flare X DDR4-3200 was selected for the capabilities of the R5 3600 which supports PCIe 4.0 which is what the X570 is all about.
Ryzen 4000 and 5000 processors will be compatible with the B550 and X570 but the existing motherboards like the X470 need a better Ryzen 2000 series CPU and chassis for dual graphics cards.