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.
"If you want every CPU to support every instruction..,"
That does indeed seem nice. As long as there's an easy way to enquire about whether an instruction is slow or not, it means that all the software runs, and the software that knows about issues with some instructions runs as fast as it would without support for those instructions. The only downside is the effort put into writing emulations of the instructions, which seems small in context.
There are two kinds of instructions that you might have or not have. The first is things such as vector instructions, crypto, string and blockmove which can usually be hidden away in library functions and you just set a function pointer to the correct version at program startup. But others such as ... just to pick a couple ... "ANDN" (rs1 & ~rs2) or "SH3ADD" ((rs1<<3) + rs2) are just naturally mixed in with potentially all of your code. The savings they provide relative to the base ISA make it simply not worth wrapping the in a library.
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.