Right now AMD AM4 motherboards support for different platforms, which include the new Ryzen 2000 processors, Ryzen 2000 G APUs with integrated graphics, 1st generation Ryzen and Bristol Ridge. For those who don’t know Bristol Ridge was AMD’s last processor generation before Ryzen, which basically introduced the AM4 socket. AMD says these chips have a lifespan beyond 2020, but according to Aanadtech many motherboard makers are reporting that they might drop support for Bristol Ridge in future motherboard releases. The reason for this is that the BIOS has to support all processors by included the microcode for them. Supporting so many CPU models makes the size of the BIOS beyond 128 megabits (16 MB), which would exceed the capacity of the BIOS flash chips used by most motherboard manufacturers. This would cause these manufacturers to use higher capacity BIOS chips, which of course cost more.
The difference between a 128 Mb flash chip compared to a 256 Mb chip is less than half actually, which motherboard manufactures do not seem to want to cover, especially considering how many people are actually using Bristol Ridge parts. This is about a $3-4 difference, which really does not sound like much, but that will definitely add up with the millions of motherboards each manufacturer will ship each year.
If you think about it most people buying an AM4 system will be using a Ryzen processor, but motherboard manufactures could release a special BIOS version that supports only Bristol Ridge and not Ryzen. That ways if you wanted to use Bristol Ridge you could easily flash the BIOS before you do your install. But then you have to think about all the people who might flash the wrong BIOS etc.