AMD recently announced that for Zen 3, it wouldn't work on older motherboard chipsets but they've taken on feedback and found a way around it.
In a post on Reddit, an official AMD employee gave a detailed answer on their new plans and they said clearly that they will be working with motherboard vendors on this. Thankfully this means for the next generation Zen 3, AMD B450 and X470 motherboards will be supported.
Our experience has been that large-scale BIOS upgrades can be difficult and confusing especially as processors come on and off the support lists. As the community of Socket AM4 customers has grown over the past three years, our intention was to take a path forward that provides the safest upgrade experience for the largest number of users. However, we hear you loud and clear when you tell us you would like to see B450 or X470 boards extended to the next generation “Zen 3” products.
Before getting too excited and popping open a bottle of the good stuff, do keep in mind it's not going to be easy and there's plenty of caveats as they outlined. Including: it will be a one-way upgrade, it will disable support for other CPUs and they plan to only offer it to "verified customers" of 400 series motherboard who have purchased a Zen 3 processor when they become available. They said it's their plan to reduce people getting into a "no-boot situation".
Obviously they're also going to only recommend the latest AMD 500 Series motherboards "for the best performance and features with our new CPUs". There's a lot of details they're working on towards this and they've started the planning with a promise of more blog style posts when the time comes closer.