Just in case it wasn't obvious (as it wasn't for me), we can add the GeomShadowText from {shadowtext} to the function `geom_sf_text()` from {sf} in place of the existing geom = GeomText argument.

geom_sf_shadowtext <- function(
  mapping = aes(),
  data = NULL,
  stat = "sf_coordinates",
  position = "identity",
  ...,
  parse = FALSE,
  nudge_x = 0,
  nudge_y = 0,
  check_overlap = FALSE,
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  fun.geometry = NULL
) {
  if (!missing(nudge_x) || !missing(nudge_y)) {
    if (!missing(position)) {
      cli::cli_abort(c(
        "Both {.arg position} and {.arg nudge_x}/{.arg nudge_y} are supplied.",
        i = "Only use one approach to alter the position."
      ))
    }
    position <- position_nudge(nudge_x, nudge_y)
  }
  layer_sf(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = GeomShadowText,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list2(
      parse = parse,
      check_overlap = check_overlap,
      na.rm = na.rm,
      fun.geometry = fun.geometry,
      ...
    )
  )
}

I couldn't easily get the example in the original question to work due to API key issues, so here's a simpler working example:

library(ggplot2)
library(sf)
library(shadowtext)
library(rnaturalearth)

Africa <- ne_countries(continent = "Africa")

ggplot(data = Africa) +
    geom_sf() +
    geom_sf_shadowtext(mapping = aes(label = name_en))

enter image description here