I can't see an easy way to find this. If you buy a board and look in the manual sometimes it is there. Searching turns up a few boards that have the settings exposed but aren't really what I'd want.
Ideally, small AMD board instead of a full sized Intel tower board. Is anyone aware of a complete list similar to, say, the QEMU one listing hardware that supports IOMMU groups? Use is multiple NVMe SSD on a x16 slot without an active switch, among other things.