Actually any problems will reflect on the motherboard OEMs themselves not AMD,so the fact some wanted to introduce PCI-E 4.0,tells me they thought their motherboards were fine. Also nothing stopping the OEMs,releasing them as BETA BIOSes(not final BIOSes),but it appears those which did want PCI-E 4.0,were forced to remove it with later AGESA versions.
The OEM B550A which is a rebadged B450 has had partial PCI-E 4.0 support for months - so trying to supress PCI-E 4.0 from the better B450 motherboards at least IMHO,is artificial product segmentation:
https://www.reddit.com/r/Amd/comments/f376m8/amd_b550_chipset_vs_b550a_b450_explained_asrock/
Gamersnexus tested one and they think there is no difference from a B450,and its internal name was B450X. Look at the chipset,it has no heatsink! Many mini-ITX motherboards such as mine,tend to have much more PCB layers,and a closer location of the CPU and chipset to the physical PCI-E slot and M2 slot. So the whole problem of signal length and needing retimers,is less of a problem in this situation,ie,there should at least be partial PCI-E 4.0 support either for the PCI-E slot or M2 slot for those motherboards.
Again,this is moving towards what Intel was doing too,and its not a good direction.
Like I said before,I hope this is some PR confusion from AMD,but if they start doing this stuff,then we can't really mock Intel too much either.