There was some rumor and speculation about some current and previous-generation Socket AM4 motherboards being able to do PCI Express 4.0 in combination with the new "Matisse" Ryzen 3000 series processors. Some motherboards were designed with PCI Express 4.0 in mind and there are even Gigabyte motherboard models that feature a PCI Express 4.0 setting in the BIOS.
AMD's Senior Technical Marketing Manager Robert Hallock clarified on Reddit that pre-X570 motherboards will not support PCI Express 4.0. Despite there being some motherboards on the market that could technically handle PCI Express 4.0, but Hallock says AMD does not want this sort of confusion in the market.
Therefore, AMD has asked motherboard makers to disable PCI Express 4.0 support on pre-X570 motherboards.
This is an error we are correcting. Pre-X570 boards will not support PCIe Gen 4. There's no guarantee that older motherboards can reliably run the more stringent signaling requirements of Gen4, and we simply cannot have a mix of "yes, no, maybe" in the market for all the older motherboards. The potential for confusion is too high.
When final BIOSes are released for 3rd Gen Ryzen (AGESA 1000+), Gen4 will not be an option anymore. We wish we could've enabled this backwards, but the risk is too great.