disabled="disabled" on a checkbox entity field
The BooleanCheckboxWidget has no settings for HTML attributes. You either have to create your own Widget or use hook_field_widget_WIDGET_TYPE_form_alter to add the disabled attribute.

Not tested personally, but you might also take a look at the Read-only Field Widget, could save you some coding: setDisplayOptions('form', ['type' =&gt; 'readonly_field_widget', ...])