PCIe x16 switching to x8 even if no 2nd GPU connected?

Have you ever thought if you can plug PCI Express cards into x16 lane slot? In this video

It's a slot of x16 size but electrically x4 This

pcle x16 slot PCIe x16: This is the largest PCIe slot with 16 data lanes PCIe x16 slots are associated with graphics cards and provide the highest bandwidth

pcle x1 slot pcie x16 slot graphics cards,4 Agree to the terms and conditions, then tap