From the commit fce63483089061379501143b58528541d0fe1fcc indicates, that the always existing 'defaultValue' property is the successor of 'selected' property.
But setting the 'defaultValue' property to true (CheckboxFieldDefinition_HideInNavigation.png) has no impact, the checkbox is not checked, and will therefore save the false value (ignoring the defaultValue).
In the past, the 'defaultValue' property was only used for defining in a group of Checkboxes and radio buttons, which one should be selected. I didn't test if this still works.
Also looking at configurations in STK, the property still exists even its not used see CheckboxFieldDefinition_HideInNavigation.png.
And in documentation the 'selected' property is still mentioned: