Together with the launch of the AMD B550 chipset based motherboards, Gigabyte and ASRock both confirmed that AMD is indeed working on Renoir-based APUs for the desktop market. These chips have been rumored for quite some time now but so far we haven't heard an official announcement. Slides from the motherboard makers reveal the Ryzen 4000G Renoir APUs will be compatible with the X570, B550 and the future A520 chipset.
Furthermore, it appears that if you install a Renoir-based APUs on a 500-series motherboard, you will not have PCI Express 4.0 support:
It is further confirmed by ASRock that Renoir APUs will be limited to PCI-Express Gen 3. This is despite the fact that B550 motherboards feature PCIe 4.0 support. AMD Renoir simply does not support Gen 4.0, so this was to be expected.