Minor correction: AMD CPUs do have something similar to efficiency cores, it's just that AMD doesn't clearly market it that way.

They bin CCDs for performance (max attainable frequency, thermal efficiency, etc) and include a good one and a not-so-good one on a CPU.

I got a 5950x a couple years ago and it's quite well known in the AMD space.

Intel's "efficiency cores" are almost the exact same thing - cheaper lamer cores to keep core count higher.