If I recall correctly, the boost "rules" are that a library submitted to boost must be conformant with the most recent version of the C++ standard. Assume the library gets accepted. Its then no required to be constantly upgraded to subsequent standards. Soooo
a) Because specifies backward compatibility as a requirement, the library is guaranteed to compile and function in all subsequent versions of C++
b) Any subsequently added library is also guaranteed to compile and function in all version of C++.
Point of pedantry: that's not true at all, stuff get's deprecated and then removed. For example unary_function is deprecated in C++11 and removed in C++17, which has just hit for example: https://github.com/boostorg/container_hash/issues/22#issuecomment-1124205780 as gcc-12 starts enforcing this (msvc has done so for a while). There are plenty of other examples. Best, John.