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A general description is here.
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 default can be selected by configuring R with ‘--with-CXX20’: the changeover is scheduled for 2026-02-08. For ways to test a package with C++20, see <https://www.stats.ox.ac.uk/pub/bdr/C++20/README.txt>.