Interestingly, there are some reports now that even some 300-series motherboards could support the Zen 3-based processors. TechPowerUp reports two enthusiasts claim they got Ryzen 5000 series chips up and running on 300-series motherboards:
An overclock.net forum user by the name Brko has recently claimed that Zen 3 processors can run on a GIGABYTE X370 motherboard with a beta BIOS and AGESA update. This claim has been partially validated by chm128256m who got a Ryzen 9 5900X to run on his modded ASRock A320M motherboard. AMD is extremely unlikely to officially support this backwards compatibility with 300 series motherboards given their previous statements. It will be interesting to see if any OEMs provide backward compatibility updates and any issues that may arise.