AMD AM5 B850M motherboards - x4 PCIe slot - open or closed

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I'm looking into upgrading to AM5 and already ordered an AMD Ryzen 7 9700X CPU. My case is a small tower so need an mATX motherboard and will probably go with a B850 one. But I have an old PCIe x8 SSD - it's physically and electrically an x8 card, but it works in an x4 slot, if it can be inserted in. That is if the x4 slot is open. My question is regarding motherboards like these:
Is the x4 slot open to allow installing an x8 card, or is it closed? Hard to figure that from the pictures. I know I can cut the plastic myself, but that will decrease the resale value when in a few years I feel the need to upgrade to a different socket.
 
Looking at the pictures (click on them again zooms in) they all look open to me. Surely you will be sacrificing performance running a 8x card in a 4x slot even if they are pci-e 4.0 slots (only the 16x slot is pci-e 5.0)? On the RS if you populate M2-3 it disables the 4x slot.
 
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