Problem/Motivation
RecipeInputFormTrait, added by #3483435: Add a trait for forms that want to collect input on behalf of a recipe, marks all recipe input elements as required when it builds the form. This generally is correct because recipe inputs are never optional. But this is a problem for checkbox elements (single checkboxes, that is, not multi-value checkboxes) because they are always present in the submitted form data, but not required to be checked. The current code makes it impossible to leave a recipe input's checkbox unchecked.
Proposed resolution
Recipe input elements that have #type => checkbox should not get the #required flag by default.