As I mention in the DESCRIPTION file, The Global Soil Information Facilities (GSIF) package is not longer available at CRAN as PROJ8 does not have the proj_api.h header. However, we needed a function to calculate the equal-area splines in our work for FAO. Therefore, we adapted the function mpsplines, wrapping it inside its own package to work with it without the burden of PROJ8. All thanks should go to GSIF authors, Tomislav Hengl, Gerard Heuvelink, and Brendan Malone.
Why didn't we work with an archived version of GSIF?
Because we wanted that all parts of our workflow to work with current versions of R.