It generally always works in practice. Common real-world example is “External GPUs” connecting via thunderbolt: these are actually just generic PCI express 4x s

It generally always works in practice. Common real-world example is “External GPUs” connecting via thunderbolt: these are actually just generic PCI express 4x slots on a cable (4x is max throughout thunderbolt 3 achieves, can’t remember what cap is on new thunderbolt 4…). Although graphics cards always generally should go in a 16x slot for best performance, they all work in the external 4x, even the latest most high end power/bandwidth hungry GPU parts. performance may suffer in some workloads though due to reduced bandwidth of the 4x slot though. You can stick any PCI express device in an external GPU enclosure, will pretty much “just work” despite being a 4x slot - these things are really external PCIe enclosures despite the GPU focus.