The point of the const parameter is to prevent you from doing the a++ !

I am pretty sure he was targeting this rule only for toplevel const. const qualified references affect the value references and are no toplevel const declarations.
A const qualifier in a toplevel declaration has no effect but documentation and belong to the implementation. In fact a toplevel const on a parameter behaves the same as:

 
    void f(int a);

    void f(int _a)
    {
      const int a = _a;
      // continue using a...
    }
 

Thanks for noticing, this is now fixed.

No. As it is copied const does not matter:

void foo(int a)
{
a++;
}
const int a{1};
foo(a);


Typo: the return type of the f() functions in the “If you declare a parameter by value const, don’t do it in the header file” section should be `int`.

Output computeValue(Input const& input, bool doCompute);

can be changed to

Output computeValue(Input const& input, bool useCaching);

without changing the definition,

It does if the passed value is truly a ‘const’ and shouldn’t be changed by the function.

I agree, I now tend to stay away from bool parameters as much as possible because they usually don’t make any sense from the call site. Is computeValue(foo, true) is using caching or not? Have to look up the interface to be sure, but with computeValue(foo, ComputeMode::UseCache) or even something even more explicit like computeValue(foo, ComputeMode::UseCacheIfAvailable) tells everything you need to know without checking the interface or having to remember it.