I don't understand - wouldn't this be observable by the person making the change? They used to call this function / use this class and now they don't any more? Even if it's inside a conditional compilation block.
in practice the exponential explosion of options may become intractable. lets say you have 10 compilation flags with 10 options each. not syre you want the compiler scanning through all that on each pass
Yes - I was only talking about when a person removes some code from use, removing the code as well as the call to the code. I wasn't meaning the compiler at all (although it would be nice if the compiler could autodetect it, as you say, that could become tricky).
That requires a person to be making that change. If the change is vibe coded and the person is only checking that the tests pass, it's not hard for cruft to accumulate.
In practice, it's annoying to track if those small util functions become dead code.