I agree with this. The code is not just a message to the compiler to build the binary/bytecode, it is also a message to other programmers. As such there is value in annotating things in that manner.

Even in Python, a very dynamic language, there is a convention that methods that start with _ are private. And I often also separate them into a different (bottom) section with a big header saying this is the private block of methods. So in that case there isn't a compiler rule but rather a common convention (which is kind of a step above comments if you wish).