Make sure your front-end code treats a missing boolean value as false.
In GROQ you can handle missing booleans and false values equally like this *[_type == 'story' && featured != true] which would match stories where featured is false or missing (or to be fair, any other value that is not true).
*[_type == 'story' && featured != true]
Sanity is a customizable solution that treats content as data to power your digital business. Free to get started, and pay-as-you-go on all plans.