You won't really find that either. I used to see that on the ultra-high end ROG boards, but it was primarily for PCI-Express x16 type slots, not M.2 slots.The fundamental aspect here isn't about disabling "m.2 slots." It's about disabling PCIe lanes. However, stating that doesn't really help. I've only been able to change the PCIe generation.
Well I already found one board (pictured in the OP) but I’d prefer an ATX option. Also the Ghost/QuartzCanyon NUCs have this option. I think Skull Canyon too but I haven’t verified that one.I've worked with and reviewed a couple of hundred motherboards and I can't think of a single one off hand that had this feature.