AMD's decision to nix official Ryzen 5000 CPU support for older 300-series motherboards has seen users pining for compatibility. Thankfully, recent developments have seen unofficial support becoming publicly available for the 300-series MOBOs anyway.
The new BIOS, spotted on jzeletronic.de (via Tom's Hardware), appears to provide support for Zen 3 processors, on these older motherboards. It should make it possible for those with an officially neglected ASRock X370 Taichi or Pro motherboard to use a Zen 3 processor, or indeed a Ryzen 4000-series CPU.
This firmware—listed as P6.61 BIOS—is most-likely an enthusiast-modded version of an existing BIOS, as previously-locked features appear to be enabled. Show caution if you are interested, though, as the first BIOS to enable X370 mobo support, this alpha is likely to be pretty buggy—as is often the case with initial iterations of firmware and software alike. And for the love of God, be careful if you decide to flash your BIOS, it can be a risky business in general, even without the chance of bugs.
This also follows a barrage of reports that users have managed to enable Ryzen 5000 support by flashing the X470 Taichi's firmware back to that of the X370 Taichi. In fact, the ASRock A320M-HDV R4.0 has been spotted on Chiphell forums supporting the AMD Ryzen 9 5900X. Provided it's running the P4.03 firmware dated November 3, the AM4 motherboards should have no issues supporting Zen 3 chips.
Currently, AMD provides full official support for Zen 3 on 500-series motherboards, with plans only to introduce support for the 400-series mobos come January 2021. But we may yet see more motherboard manufacturers sneaking out features they're not officially supposed to, as is tradition. Perhaps this P6.61 BIOS is the sign of things to come. Maybe wait for more people to test it out first—let the internet be your guinea pig.