The rumours only point to it being AM4, if you believe those rumours even:
https://www.hardwareheaven.com/amd-n...d-of-the-year/
But B450? AMD folded under pressure with the 5000 series, but toss a coin on whether they will allow another series on the old platform. At this point there are enough reasonable cost B550 boards out there and we must be due a B650 at some point. If there really are CPUs and APUs using Zen3+ then that's quite a few old devices need to be removed from support in the AGAESA along with the already killed off constriction core Athlons and I think the original Zen parts. I'm guessing at that point B450 would support 3000 series and later, ie exactly what B550 supports. Which I guess is where the whole B450/B550 support rift came from
That chart where these rumours seem to come from, the top of the range Raphael having RDNA2 support rather than being a CPU... I don't know whether to be fascinated or just cry fake at the whole slide!