Despite claims that Ryzen 5000 processors wouldn't be supported on 300-series motherboards, screenshots of a Ryzen 9 5900X running on an A320 motherboard have been spotted on a Chinese forum.
AMD has only officially announced support for Ryzen 5000 processors on 500-series and 400-series motherboards, leaving 300-series motherboard owners out of the loop. According to AMD, the reason behind not adding support for older motherboards was because some motherboards didn't have enough flash memory to store all the necessary data to include support for all the new AMD processors. Back when the Ryzen 3000 launched, some vendors worked around this issue by removing some GUI elements from the BIOS to free up space for the BIOS updates.
Image credit: Chiphell
As per the images shared on Chiphell (via TechPowerUp), it's possible to run a Ryzen 5000 series processors on a 300-series motherboard. The images show a Ryzen 9 5900X running on a modded ASRock A320M motherboard. Additionally, Brko on Overclock.net claims to know someone that has a “beta BIOS with AGESA code that supports Zen3” for an X370 Gigabyte motherboard.
Taking AMD's statement regarding the Ryzen 5000 series motherboard compatibility into consideration, there's a big chance that these BIOS were modded. This means that they won't be supported by the motherboard vendors, but if they get a public release and all goes well, you will be able to run a new Ryzen 5000 processor on your old 300-series motherboard, which is pretty neat.
KitGuru says: Do you have an AMD 300-series motherboard? Would you like to receive support for the Ryzen 5000 processors on your motherboard?