The quotation below is seen at cppreference:
std::strstreambuf
Defined in header <strstream>class strstreambuf : public std::basic_streambuf (deprecated in C++98)
What does the (deprecated in C++98) indicate?
Does it mean that the above constructor of std::strstreambuf is deprecated to be used in C++98 and afterwards?
If that's is true, it seems that std::strstreambuf also should not be used since there is only one constructor for std::strstreambuf and you have no other choice.
As far as I can see, it has nothing to do with the C++ attribute: deprecated.