Note that SFML is an open source library, and they have this issue ;-)
I do have to say that your article is kinda limited to "the exact scope of your project". Even if it hits quite a few valid points.
Also, about const, adding "const" can be important for other reasons then just API. It also impacts performance (as a result from a const function can be re-used by the compiler in certain cases). And occasionally what you are allowed to do (if you deal with rvalue temporaries) And also, there is no guarantee that a const does not modify an object, I've seen something along these lines:
Matrix4 getTransformationMatrix() const
Where a local cache is updated in a const function. It's const in the fact that it returns the same value every time, but it's not const in that it doesn't modify internal state and thus not thread safe for reading. I would never do this, but I've seen it.