Faster DDR4 won't be supported by motherboards/IMC, so there is no point in manufacturing any higher DDR4. What we actually see is that the fastest RAM is not even manufactured anymore. I mean 5000+ kits from most brands are not manufactured and, in most cases, not available in stores. What is left seems to be from older production.
ADATA, V-Color, and some others had DDR4-5600 kits based on Hynix DJR. You can see them listed on the QVL of some motherboards. However, they were barely available. In stores was easier to find something like that Team Group or Kingston at 5333.
On Intel was hard to stabilize anything above 5200. On AMD 5000, the non-APU max was about 5000. For 4000/5000 APU, it was up to 5400-5600, depending on the IMC. In short, even 5333 kits seem to be pointless, as you can find maybe 10 motherboards in total that support this speed, and you still need a CPU that handles it.
Every generation is getting faster only to some point. When problems with stability at a higher clock start, then the capacity goes up. When the new generation is close to release, then the max speed available in stores is going down (more expensive for production, so there is no point in doing it when the faster gen is out). The new gen is more profitable, so there is no point in wasting time and money on the older gen.