How can I propagate const when returning a std::vector from a const method?

In C++20, you can just return a std::span with elements of type const int*

#include <vector>
#include <span>

class Data
{
public:
  std::span<const int* const> getIndex() const { return this->index; }
private:
  std::vector<int*> index;
};

int main() {
  const Data data;
  const auto index = data.getIndex();
  *index[0] = 5;  // error: assignment of read-only location
}

Demo