This blog is updated daily..
A general description is here.
The transitional support for installing packages without namespaces (required since R 2.14.0) has been removed. ‘R CMD build’ will still add a namespace, but a ‘.First.lib()’ function will need to be converted.
‘R CMD INSTALL’ no longer adds a namespace, and a ‘.First.lib()’ function in a package will be ignored. [The latter as from ca Dec 21.]