There is a layer constructor function called borders(). It draws geographical borders. While it is not a bad name for a function on itself, it doesn't really "fit" in the established grammar of ggplot2. For example, it is not immediately clear that borders() is dissimilar to a helper like margins() or is unrelated to gt::cell_borders(). It isn't on face-value related to any of the other grammar components, so it is not intuitive that it can be +-ed to a plot.

I think it should become something like annotation_borders(), where it is at least clear that it falls with the 'annotation'-flavour of layers.