OK, I completely disagree with my statement:

"""If you added this as an argument to shutdown() then you'd probably also have to add it as an option to the constructors (for people using Executors as context managers). But, if you have to add it to the constructor anyway, you may as well *only* add it to the constructor."""

This functionality wouldn't apply to context manager use (since the point of the context manager is to ensure resource clean-up). So maybe another argument to shutdown() would make sense