I won't speak for cafkafk, but I have two E5 (v3/v4) systems one on DDR4 and one on DDR3. This generation of CPU all support DDR4, but a few skus do support DDR3 also. ChatGPT told me they were niche products to meet specific customer needs.

I just picked up the DDR3 board, an Aliexpress "XD3" so I could reuse some DDR3 ram on a better CPU. Quad channel 1866MT/s is not bad!