To cite extrafont in publications use:

  Winston Chang and Frederic Bertrand (2025). Tools for Using Fonts, R
  package version 0.20. doi:10.32614/CRAN.package.extrafont.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {Tools for Using Fonts},
    author = {Winston Chang and Frederic Bertrand},
    publisher = {manual},
    year = {2025},
    note = {R package version 0.20},
    url = {https://github.com/fbertran/extrafont},
  }