[ignoramceisblis]

It's not new (as 2 other commenters posted). See for example (DDR4 on X470): https://www.asrock.com/mb/AMD/X470%20Master%20SLI/index.asp#... and also (DDR4 on X570): https://www.asrock.com/mb/AMD/X570%20Steel%20Legend/index.as...

Scroll down to the tables listing UDIMM Memory Slot and you can see how different configurations of single rank vs. double rank play out with respective to what becomes the "max" frequency.