This organization was used to host static libraries to build R packages up till R 4.2.
For R-4.3 we switched to a new system to create such bundles that also supports the new arm64 hardware, see here:
- Sources: https://github.com/r-windows/ucrt-libs
- Binary bundles: https://github.com/r-windows/bundles/releases
Also, these days rtools43 itself includes a lot of libraries which can be used to build some R packages on Windows.