Internally within MongoDB we have several scripts that make use of the legacy shell and parse its output. The scripts use the --quiet command line parameter to stop the shell from intermixing log output with the output from javascript execution.
The behavior of --quiet is fairly brittle and is easy to unintentionally break. To help mitigate this, it would be useful to add a simple Javascript test that runs the shell with --quiet and ensures that the output contains no extra log lines.