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A general description is here.
The default C++ standard has been changed to C++14 where available (which it is on all currently checked platforms): if not (as before) C++11 is used if available otherwise C++ is not supported.
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 Extenions’) 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)
Subsetting/indexing, i.e. ‘ddd[*]’ on a ‘DOTSXP’ (dot-dot-dot) object ‘ddd’ has been disabled; it worked by accident only, undocumentedly.