Does a cooler like the NH-D15 block RAM on DDR5 motherboards?

It depends on the specific motherboard, not the memory type. DDR5 sticks are physically the same size as DDR4, so the clearance issue is about how far the heatsink overhangs the DIMM slots. Some DDR5 boards shift the socket or DIMM slots closer, but it’s rare. Check Noctua’s compatibility list before buying, or work around it by moving the front fan up or using only the middle fan.