According to the formslib docs (https://docs.moodle.org/dev/lib/formslib.php_Form_Definition#modgrade), the fourth option passed to a new modgrade element should be a boolean that controls whether a "No Grade" option is listed.

However, looking at the code in lib/form/modgrade.php, the list of options seems to be set the same way every time, and indeed it doesn't seem to make a difference if I pass true or false to addElement().

If the docs are out of date, I'd like to advocate for having the option reinstated.