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!
There were several V4 Xeon models that supported DDR3 AND DDR4 simultaneously. If you had a motherboard with an X79 chipset it would (sometimes) work properly.
I am not aware of any commercial vendor shipping v3/v4 boards with DDR3. I have a couple hundred Supermicro systems that are stuck on v2 CPUs with DDR3...
This also means that you need to know the processor your motherboard supports (or, easier, probably RAM) before putting in an order to upgrade the processor. (These processors are incredibly cheap, less than $10 for something that might have cost literally thousands ten years ago, so worthwhile to spend a few minutes and pick out your favorite based on cores, watts, Ghz, etc.)
(Another commenter says that there are some motherboards that accept v3/v4 but also can run slower DDR3 RAM. That's new to me and quite cool - DDR3 is extremely cheap, even now. I did find these motherboards on aliexpress, too: https://www.aliexpress.us/w/wholesale-XD3-motherboard.html?s... and one clearly says v3/v4 cpu's with DDR3 RAM. That could be very useful although memory speeds are slower since CPU performance can be boosted with v3/v4.)
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!