Earlier today, AMD launched their latest Ryzen 3000 series of processors. One of the major changes is that the processors are now capable of supporting PCIe 4.0 lanes.
Technically speaking, if you were to run the new Ryzen 3000 processors on an older compatible board like the X470 chipsets motherboards.
AMD is suggesting that manufacturers do not enable it to avoid instability to the overall system.
This is largely because the older X470 design was meant for PCIe 3.0 and not PCIe 4.0. It remains to be seen which manufacturers will want to experiment or open up this option. Everything will be clearer by 7th July 2019 when the processors and motherboards are officially launched.