Why would you only use 3200 or 3466 on a B450 & X470?If you're using B450 or X470 boards with the 3950x, only use 3200mhz or 3466mhz kits at most.
If X570 go with a standard 3600 CL16 kit. However stuff like CL18-21-21-40 aren't that bad. You can go CL18 and be fine. It's just the CL16 is mostly recommended because it's popular and it's the fastest 3600mhz latency around right now. Basically has similar latency to 3200mhz CL16 which isn't bad.
Why would you only use 3200 or 3466 on a B450 & X470?
I’m not sure if this is unusual but the B450 Tomahawk Max claims to support up to 4133mhz https://www.msi.com/Motherboard/B450-TOMAHAWK-MAXMuch higher chance of unstable RAM at 3600. The nice thing with X570 is that the memory tracing is much improved over older boards, allowing super high frequencies like 3600-4200mhz.
Basically all B450 boards and most X470 boards don't even support 3600mhz on the DIMM slots themselves. (Of course this doesn't mean it WON'T run, just means they aren't rated for it.)
So you can run 3600mhz for sure on previous gen mobos, but much higher chance of it not working and you having to downclock to 3466mhz.
And 3600MHz CL16 isn't even that great of a performance bump over 3200MHz CL16, so gambling that 3600mhz will work on B450/X470 for a 3rd gen CPU in my opinion is just a waste of time.