New alleged evidence (via Harukaze5719) has emerged showing that it may be possible to get AMD's Ryzen 5000 (codename Vermeer) processors to work on older 300-series motherboards. Currently, only 500-series and certain 400-series motherboards officially support the new Zen 3 chips.
A Chiphell forum user posted screenshots of a Ryzen 9 5900X running on ASRock's A320M-HDV R4.0 motherboard with a modified firmware. The version is blurred out so it's unsure if the user altered the firmware or ASRock provided it.
The A320M-HDV R4.0 is a budget motherboard that retails for $64.99 and features a rather modest six-phase power delivery subsystem. However, the A320M-HDV R4.0 does support the Ryzen 9 3950X. Technically, it should be able to handle the Ryzen 5000 chips without a sweat. Logically, PCIe 4.0 is disabled due to the nature of the motherboard. Whether the processor works flawlessly or not is uncertain since the forum user didn't provide any comment or benchmarks.
Another user from the Overclock.net forums claims to have irrefutable evidence of Zen 3-based processors functioning on a Gigabyte X370 motherboard. The motherboard is reportedly on a beta firmware with AGESA code that supports Zen 3. Once again, the firmware's origin is unknown. However, the forum user alleges that other than the lack of PCIe 4.0 support, the processor is operating routinely. He has yet to provide any proof, but vowed to share screenshots as soon as he gets them.
The capacity of the SPI ROM chip has been one of the main limiting factors that have prevented previous generations of AM4 motherboards from supporting the latest chips. AMD launched the AM4 socket back in 2016. Since AM4's introduction, a plethora of processors, including APUs, families have gone through socket, making it a challenge to fit support for every single Ryzen chip on the small SPI ROM chip. Even with the 400-series motherboards, vendors will have to make certain compromises to support Ryzen 5000, such as eliminating support for older Ryzen processors. In the past, some vendors have even killed the fancy GUI and disabled certain features to add support for new chips.
If the 300-series motherboards end up getting support (official or unofficial) for Ryzen 5000, owners will likely face the same limitations as 400-series owners.