constexpr member functions that don't use this?

Both programs are well-formed. The C++14 standard requires that s.f() be a constant expression because it is being used to initialize a constexpr variable, and in fact it is a core constant expression because there's no reason for it not to be. The reasons that an expression might not be a core constant expression are listed in section 5.19 p2. In particular, it states that the evaluation of the expression would have to do one of several things, none of which are done in your examples.

This may be surprising since, in some contexts, passing a non-constant expression to a constexpr function can cause the result to be a non-constant expression even if the argument isn't used. For example:

constexpr int f(int) { return 42; }

int main()
{
    int x = 5;
    constexpr auto y = f(x); // ill-formed
}

However, the reason this is ill-formed is because of the lvalue-to-rvalue conversion of a non-constant expression, which is one of the things that the evaluation of the expression is not allowed to do. An lvalue-to-rvalue conversion doesn't occur in the case of calling s.f().