This is the reproducible example.

if (!require("data.table")) {
  install.packages("data.table")
  require("data.table")
}

dat <- data.table(a = 1:3)
dat[, b := a + 1]
message("No print above expected, but will occur if you source this script.")

I am suppressing this print by piping it to invisible as follows:

dat[, b := a + 1] |> invisible()

If someone has a better solution, please let us know.