Selective reading at it finest.
The past showed us that bios memory was an issue, with some board from MSI not being able to accept every AM4 CPU, or even some functionality like sata raid when it's supposed to be a standard. It was already a mess because it means that some brands had full am4 compatibilty, when some other had a selective compatibility.
Now AMD is saying because of that same reason (aka motherboard makers being cheapstakes) they literally can't have every single am4 motherboard be compatible with every am4 cpu. Ryzen gen 1 was scrapped from having official support for 500 chipset, but...some x570 motherboard are compatible with gen 1 ryzen :
The issue ? not all board might be able to handle that because again, all manufacturer don't use thet same bios chip. So you can't just say that x570 support is official when it's going to be a case by case possibilty. That's just a nightmare for the consumer.
And you can't either start to sell some b450 with a bios only compatible with zen 2 and zen 3. Some guy might still want to get a cheap zen+, another one with zen 1 might have his old board dying on him, but suddenly a few b450 dropped support for his cpu, so you gotta look wich brand, wich model can support your cpu...
Then you get that guy with his shiny new ryzen 4000 cpu, and shiny new* b450 motherboard...who won't work because it's one that use a small bios chip.
It's a mess, but not a mess where amd is the only one at fault, motherboard makers made is so that full compatibility with every AM4 motherboard isn't possible across the board, without introducing some hellish headache when you wonder if x board will work with x cpu, because of all the different bios running around.