Most system builders and tech support reps don't even think about this as an issue. The "topology" of the motherboard is very important to know when there's a buggy problem and RAM is even in the question. From whether one can run 4 sticks of anything to only 2 sticks, it boils down to the topology of the motherboard more than the CPU's memory controller.
Here's a good explainer: Buildzoid Rambling about motherboard memory layouts : hardware (reddit.com)
You can lookup any board to find out what type of topology it has for the most part. It usually won't be on the manufacturer's website. It will be on a forum, Reddit post or online somewhere. A good indicator is about 90% of your B450's and lower are mid-range boards with "daisy chain" memory. Most premium boards with the X470-570 chipsets are "T" topology.
With "T" topology there's more RAM compatibility and nearly no issues running 4 sticks at a once. With "daisy chain" topology there's limitations on what speeds and brands of RAM one can run in any given board using all 4 slots. However, running just 2 slots there's less issues but still some compatibility and overclock issues that can arise.
So while the memory controller is on the CPU die in today's processors, the topology of the motherboard will play a huge part in how RAM is handled. Before we say it's time to RMA the CPU, look very closely at other options. This topology thing isn't new, it's just largely forgotten.
"It worked before you broke it!"
