Motherboard makers are taking no chances with the upcoming B550 chipsets. Most boards, even the cheapest models are going to support all four generations of Ryzen CPUs, plus the accompanying APU lineups based on the same core architecture. This was confirmed by a bunch of BIOS pictures showing the ASRock B550 Phantom supporting chips across all three generations and the accompanying APUs as well:
As you can see, the Ryzen 7 1700 from the 1st Gen lineup, the Ryzen 7 2700X from the 2nd Gen lineup and the Ryzen 7 3700X from the 3rd Gen lineup are all shown running on the same B550 motherboard. Not only that but the Ryzen 5 2400G (Raven Ridge APU) and Ryzen 5 3400G (Picasso) also seem to be supported by the B550 Phantom Gaming board. This is quite spectacular considering that the board in question is a $115 product and yet supports such a wide range of products.
The reason behind this is that many B550 motherboards come with two, not one 32MB BIOS ROM chips, each partitioned into two 16MB sections to support the older processors. Earlier AMD had to backtrack its decision to drop support for the 4th Gen Ryzen processors from the 400-series boards after widespread backlash from fans and enthusiasts alike: