I think it is probably an oversight and not intentional that those are not there:

constexpr std::string operator +(std::string const &, std::string_view const &);
constexpr std::string operator +(std::string_view const &, std::string const &);
constexpr std::string operator +=(std::string &, std::string_view const &);

I would even hope for this one:

constexpr std::string operator +(std::string_view const &, std::string_view const &);

I agree that implicit conversion from a view back to std::string invokes the danger
of accidentally enabling an expensive std::string construction, but in this case
I think it is clear that the operator must create a std::string instance.