Masking (i.e., not displaying the literal typed text
as input) is supported on most, but not all
platforms. It is supported in RStudio, provided you
have a suitable version of the GUI. It should also work in the
terminal on any major OS. Finally, it will work in any environment
where the tcltk package is available (e.g., Windows with RGui).
Notably, this will not work with Emacs; passwords can be read, but
there will be no masking.
In the terminal, the maximum length for input is 255 characters.
Additionally, messages printed to the terminal (including the
"*" masking) are printed to stderr.