There has been said and written a lot about PCIe Gen 4.0 support on Series 300 and 400 chipset based motherboards from AMD.

One of the first companies that created a viral in an effort to draw attention was Gigabyte, who introduced a Firmware BIOS switch in their X470 motherboards allowing you to enable PCIe Gen 4.0 (partially). Once that happened a lot of motherboard manufacturers felt obligated to follow. Meanwhile, AMD has been stating that non-500 series motherboards will not get PCIe Gen 4.0 support. 

Check out what is happening at the support pages of Gigabyte, they are now removing PCIe Gen 4.0 support from their products starting with the new AGESA 1.0.0.3 ABB BIOS update. Bios version F42a and later is specifically removing support. Gigabyte is listing the withdrawal of the feature with chipsets from X370 and X470 to B350, B450, and A320.

As to why support is removed is not explained.

Gigabyte removes PCIe 4.0 support on all AMD motherboards that are not X570