AMD Ryzen 4000 Zen 3 will be compatible with Socket AM4, but it is end of the road for X470, B450, and below motherboard owners [notebookcheck.net]
AMD's Robert Hallock has confirmed that the upcoming AMD Ryzen 4000 Zen 3 processors will be compatible with Socket AM4 as long as the motherboard features an X570 or B550 chipset. Zen 3 will not support older chipsets owing to lower space on the EEPROM of these motherboards. Future prospects of Socket AM4 depend on the industry's I/O innovation.
[...] In a blog post, Hallock confirmed that current AMD X570 and B550 chipset motherboards will support Zen 3 processors after a BIOS update. However, Zen 3 processors will not be compatible with any chipset prior to X570 or B550. This means end of the road for all those who have X470, B450 and below chipset boards. Hallock says that this decision had to be taken as due to BIOS capacity limitations on older platforms.
We've seen AMD taking a similar stance with Zen 2 as well by removing drop-in support for motherboards that have just a 16 MB EEPROM. X570 motherboards have a 32 MB EEPROM thereby enabling larger a AGESA codebase to be comfortably accommodated.
AMD blog post [amd.com].
See also: AMD axes Zen 3 support on 400-series motherboards: Is AMD pulling an Intel? [techspot.com]
AMD will use the AM4 socket through its 'Zen 3' CPUs, but it will drop older chipset support [pcworld.com]
B450 and X470 chipsets won't support AMD Ryzen 4000 processors [pcgamesn.com]
Hardware Unboxed: No AMD Zen 3 Support on 400 and 300 Series Motherboards [reddit.com]
AMD Zen 3 Based Ryzen 4000 'Vermeer' Desktop CPUs Will Be Compatible With Existing AM4 (X570, X470, B550, B450) Motherboards, Confirmed By XMG [wccftech.com] (April 16, fake news or specific to manufacturer?)