How to extend Views Plugin ViewsSelection.php to have $form_state
There is no form state, at least not with autocomplete. And since the same code is designed to work with autocomplete and other widgets, it can't rely on form state anywhere.

If you have autocomplete you can possibly pass in the settings in the entity_autocomplete form element which should be passed to your plugin. Not sure about other widgets, you might need your own widget to be able to do that.