I've noticed how in C++11, C++14 and onward, more and more Boost libraries are adopted/incorporated into the actual language standard (or into TS documents, which are likely to end up as part of the standard): Boost.optional, Boost.Any, Boost's threading library, smart pointers, etc. etc.
Are these libraries now maintained solely for usage with C++ code using an older version of the language standard, or do they have additional use? Specifically, are some of them considered alternative semantic variants to the choices of the C++ standards bodies?