As a more modern example, pdep & pext (BMI2) on Zen 2 and Zen 3 are extremely slow, even though they're technically supported. Thus any code wanting to actually use those for anything is essentially required to add a "But actually! If I'm running on Zen 2/3, pretend pdep/pext do not exist".

If you want every CPU to support every instruction without caring about perf, you could just have the OS emulate it on hitting illegal-instruction; perf isn't gonna be that different given that nothing would be using any of the potentially-slow instructions anyway and the ISA might as well not even have it in the first place. It just solves absolutely nothing to technically support everything.