Should I always define a constexpr static data member outside of the class?

Why a constexpr static data member needs to be defined outside of the class on C++11, C++14 but it doesn't need that requirement on c++ 17, 20 and above?

struct Array{
    int static constexpr sz_ = 5;
    int ar_[sz_]{};
};

//int constexpr Array::sz_; // needed on `C++11`, `C++14` but not needed for C++17, C++20

void foo(int const& x){

}

int main(){
    foo(Array::sz_);
}

• So is this a change in the standard? or should I always define that member outside of the class?