This blog is updated daily.
A general description is here.
The default C++ standard will be changed to C++20 ‘where available’ (which it is on all known platforms from 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 esssential.)