Surprise! Turns out that AMD’s 4th Gen Ryzen desktop processors won’t work with existing B450 and X470 boards after all. You’ll need an X570 or B550 based motherboard to run the new Vermeer chips. (sunnyflowerdelivery.com) Meanwhile, the Ryzen 3000 processors will support both B550 as well as the B450 chipsets.
The B550 chipset brings faster I/O options to the masses. You get four 10Gbps USB ports, one x16 PCIe 4.0 slot for discrete graphics cards, one PCIe Gen 4 slot for NVMe storage, plus SLI/CF support with B550. The only difference between this and the X570 platform is that the general-purpose lanes such as the x4 and x1 slots will be limited to PCIe 3.0.
Update: AMD explains the reason behind this in the following blogpost:
AMD has no plans to introduce “Zen 3” architecture support for older chipsets. While we wish could enable full support for every processor on every chipset, the flash memory chips that store BIOS settings and support have capacity limitations. Given these limitations, and the unprecedented longevity of the AM4 socket, there will inevitably be a time and place where a transition to free up space is necessary—the AMD 500 Series chipsets are that time.