AMD has officially stated that Ryzen 5000 support for the 400 series chipset will be coming in 2021, but that doesn't appear to be the case anymore. Three companies have shared (so far) that they will support AMD's new Ryzen 5000 series CPUs before 2021, Biostar, Asrock, and Asus. With Asrock already having beta BIOS's ready for the shiny new CPUs.
To ensure compatibility, you'll need a BIOS for your specific B450 motherboard that supports AGESA 126.96.36.199 at the very minimum. This AGESA code enables Ryzen 5000 Renoir compatibility. In order to get the full performance out of your Ryzen 5000 CPU you'll need to make sure your B450 motherboard has a BIOS supporting AGESA 188.8.131.52 or greater.
Biostar has said via Twitter that they will be supporting Ryzen 5000 CPUs coming soon, whether that's before 2021 or after 2021 we are not sure. Regarding Asus, they are more straight forward with the deadline, marking a December release for AGESA 184.108.40.206 BIOS's that will be available for all its 400 series motherboards (not just B450).
Asrock is the strangest of the three though; reportedly Asrock has released beta BIOS's for its entire B450 lineup of boards (no word on X470), but quickly withdrew them from the public eye. AnandTech covered the strange occurrence and they have no idea why the BIOS updates were pulled.
As for the remaining motherboard manufacturers like MSI, and Gigabyte (to name a few), they have kept quiet on the matter. Presumably, these remaining companies will be waiting until Q1 of 2021 -- as AMD has said (which is the official deadline) -- before releasing new BIOS's for the Ryzen 5000 CPUs.
So for now, there are still no official BIOS updates for B450 motherboards that support the new AGESA codes and thus the new Ryzen 5000 processors. But fortunately, it looks like Asrock, Asus, and presumably, Biostar will offer support before 2021.