I would think that all (most?) should be able to run it fine. Intel did this before with the move from PCIe 2.0 to 3.0 between Sandy and Ivy Bridge. Both were compatible with the board, but you got 2.0 if you used Sandy Bridge and 3.0 if you used Ivy Bridge. I think the plan is definitely to move to PCIe 4.0 with Rocket Lake so compatible boards which are released now should be compatible with the next gen features.
Perhaps that's the reason of the price.
Personally buying a mobo these days with out PCI4, high end, I think it's silly.
You can argue there's no performance gain on the GFX, but once you are talking about storage, they always benefit, even more going forward.