Apologies if I'm missing something but it looks like ggplot2 has not been updated on cran for ages. The version on CRAN is 2.2.1, pub date 2016-12-30 https://cran.r-project.org/web/packages/ggplot2/index.html
As a result, CRAN is out of sync with the ggplot2 documentation: http://ggplot2.tidyverse.org/reference/ggsf.html
I see a solution here:
But I would like to know when ggplot2 will go to CRAN?
Solely to close the loop ggplot2 3.1.0 was released on 2018-10-24 and geom_sf will retire fortify before long.
ggplot2 3.1.0
geom_sf
fortify