None of this stuff is used anymore. We have the kernel in ring zero and we have everything else.
Not using two of the four rings does not really mean nothing is used anymore. The reason only two are commonly used is probably to a large extend due to portability to processors with only two rings and maybe also architectural simplicity.
Even x86 barely supports rings 1 and 2. The modern (386+) paging system only recognizes two privilege levels, and the fast privilege change instructions (SYSCALL, SYSRET, etc) are only useful when switching between rings (really "CPL") 0 and 3.
If you're programming a 286, then you can go whole hog with 4 rings.
In essence, the original concept of fixed number of rings that not only can, but have to, be distinguished by code runing in them is why there have to be hackish more-than-ring-0 modes like SMM.
Probably architecturally cleanest solution involves having only two modes with privileged operations in one affect hardware directly and in the other all such operations trap in a way that can be emulated. Interesting variation on this concept is having only simple trap and interrupt dispatcher running in the privileged mode with OS kernel being run in user mode as seen by hardware (Alpha does essentially this, but the privileged code depends both on hardware and operating system, so you don't get the benefits of easy virtualization).
Sure. My point was more that the overwhelming majority of the privilege separation implemented in modern SoC's has absolutely nothing to do with a 33 year old "ring" model and we should stop using that term. It hurts more than helps.
Not using two of the four rings does not really mean nothing is used anymore. The reason only two are commonly used is probably to a large extend due to portability to processors with only two rings and maybe also architectural simplicity.