What happens to Boost libs after their inclusion in C++, other than usage with older-standard code?

I've noticed how in C++11, C++14 and onward, more and more Boost libraries are adopted/incorporated into the actual language standard: Optional, any, threading, 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?