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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).
Supoport 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’.
The default C++ standard will be changed to C++20 ‘where available’ (which it is on all known platforms since 2021): if not C++17 is used if available otherwise C++ is not supported (as before).
Several CRAN packages already require C++20 and have hundreds of others depending on them. (Packages can request C++17 if essential.)
For now this can be selected by configuring R with ‘--with-CXX20’: it is planned that this will become the default before release.