This blog is updated daily.
A general description is here.
Specifying C++11 or C++14 in ‘src/Makevars’ has long given a note: in 99% of CRAN packages the specification was unnecessary as the default C++17 sufficed (in a few cases with a deprecation warning from some compilers).
Support for these standards has been removed: the default C++ standard will be used.
‘R CMD config’ variables ‘CXX11’, ‘CXX14’ and their associated ‘CXXxxFLAGS’, ‘CXXxxPICFLAGS’ ‘CXXxxSTD’, ‘SHLIBCXXxxLD’ and ‘SHLIBCXXxxLDFLAGS’ variables are no longer supported and reported as ‘defunct’.