Said someone that never researched miscompilation issues or UB.

They are mostly deterministic.

LLMs can also provide the equivalent of an -S switch.

You are correct, of course, but the general consensus is that (in languages other than C++) compilers are largely close enough to deterministic that the difference doesn't matter. LLMs, by design, don't even attempt to go in this direction.

Not entirely sure what I'd do with an LLM -S switch. Debug the output/intermediate representation of the LLM, knowing that the next time I press "enter", it's probably going to give me an entirely different output?

It won't be much different from human computers in that regard, isn't after all what everyone is looking for as replacement?

Probably, yes.

But then, aren't we all happy that we moved from human computers and to electronic computers with programming languages and compilers?

The first is a bug in the compiler, the second is a bug in your code.

Not everyone agrees, see Linus.