UCRT3 Experimental R-devel for Windows uses this experimental version of Rtools (currently an experimentat version of Rtools43) for Windows.
The experimental Rtools installer is available here.
More information about UCRT3 is available here.
One can also download the base toolchain tarball, the full toolchain tarball and the cross toolchain tarball.
GitHub actions may be used to install R, a tarball variant of Rtools, and to check packages using these GitHub Actions package checking resources.
Sources are available for the toochain tarballs and the Rtools installer.