It still documents intended behavior, though. C++ is basically a giant clusterfuck if you want the compiler to actually enforce anything - because of pointer arithmetic, there's no guarantee not only that const Foo& a is immutable, but even that it points to a valid Foo or that accessing it won't crash your computer. It's trivially easy to trigger undefined behavior that may do anything up to pwning your computer, whether you make your classes immutable or not. The point of const, access control modifiers, references, static typing, RAII, and all of the other restrictions on C++ programming is to put up giant signposts that say "Don't do that! Here be dragons!" rather than actually prevent you from doing it.

If you want the compiler to actually enforce safety properties of the language, use a language like Haskell or Rust.