The goal of geoproj is to make PROJ 7.0.0 available to R packages.
You can install the development version from GitHub with:
# install.packages("remotes")
remotes::install_github("paleolimbot/geoproj")
Right now, this package barely compiles (but does!).
library(geoproj)
proj_version()
#> [1] '7.0.0'