rClr has been ported to work with R 3.5.x. Support for R 3.5.x is enabled by the use of R.NET 1.8.1. The release includes two windows binary packages for R 3.4.x and 3.5.x.
There is no tarball downloadable for building on Linux in this release; an ulterior release should soon have a tarball once a few issues with the newer mono build toolchain are sorted.