If you were hoping to buy a new 3000 series Ryzen CPU and add PCIe 4.0 support to your existing motherboard, that dream is now well and truly dead. Here is the quotation from rhallock in a post on reddit.
"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."