Interesting to see this show up literally the day after I post a comment with this 11-year-old article on the same topic:

https://www.extremetech.com/extreme/188396-the-final-isa-sho...

It is a little ironic to see AMD making the backwards-compatibility argument, when they were the ones who made the very inelegant AMD64, had some known differences to Intel's CPUs in some edge-cases, and then much later, https://www.os2museum.com/wp/vme-broken-on-amd-ryzen/

There are some very-low-power x86 SoCs which are largely found in embedded systems; the most famous of these may be https://en.wikipedia.org/wiki/Vortex86