So, I'm not sure where you're looking or what you're seeing, but the X570 Gaming X revision 1.0 doesn't have a "F31b" BIOS version available. The latest version available for that board is version F30, and that is the version I'd recommend that you install.
https://www.gigabyte.com/Motherboard/X570-GAMING-X-rev-10/support#support-dl-bios
I don't see any mention of truncating support for any previous generation CPUs in any of the BIOS notes, which I didn't expect to anyhow. The only boards I've seen that have had reduced CPU support when updating to newer versions have been SOME (Not even all) of the B450 and X470 boards, and numerically a higher number of the older B350, X370 and A320 boards. And that was a Ryzen 3000 series support thing, it had nothing at all to do with Ryzen 5000 series.
None of the older Ryzen chipsets like B350 will support Ryzen 5000 series, no matter what. And all or at least most of the B450 and X470 chipsets WILL support Ryzen 5000 series, and I've not seen any indications that there is going to be an elimination of older model CPUs in order to accommodate them. At least not so far. B550 doesn't support the 1st and 2nd Gen CPUs anyhow, at all, and the X570 chipsets should support all of them from first gen to 4th gen. I've seen nothing that says otherwise.