This blog is updated daily.
A general description is here.
Packages which specify C++11 will still be installed using C++11.
C++14 compilers may give deprecation warnings, most often for ‘std::random_shuffle’ (deprecated in C++14 and removed in C++17). Either specify C++11 (see ‘Writing R Extensions’) or modernize the code and if needed specify C++14. The latter has been supported since R 3.4.0 so the package's ‘DESCRIPTION’ would need to include something like
Depends: R (>= 3.4)