Note I haven't tried this so the following is opinion, however this should only be the issue with the 400 series boards (it has to do with the older boards bios having limited capacity - which is becoming a problem now so many different cpu's can fit to the same socket). I also think the new bios should still support 3000 series cpu's - its the Zen / Zen+ based parts that may get removed in order to make room for the new cpu's.
With respect to 'no going back' again - I think this may depend on the board, boards with the ability to flash the bios from a USB stick without a CPU installed can probably be reverted to whatever bios the user wants to. Hardware unboxed used an X570 motherboard and were able to test Ryzen 5000, as well as cpu's from the 3000, 2000 and even 1000 series all on the latest bios (note the 1000 series cpu's aren't officially supported on X570 but worked fine anyway).