A maintenance release RcppClassic 0.9.9 is now at CRAN. This package provides a maintained version of the otherwise deprecated first Rcpp API; no new projects should use it.
Per a request from CRAN, we changed the call to
Rcpp::Rcpp.plugin.maker() to only use
:: as the function has in fact been exported and accessible for a pretty long time. So now the usage pattern catches up.