The “not enterable” and “not modifiable” field properties have been removed in the conversion to project mode. The enterable property is converted to a form object property where the field is used on a form. The exception is Boolean fields - they are now enterable and enabled on every form.
There is not a form editor option to make the boolean field disabled by default. So it seems the only way to fix this is add code to every every form where the boolean field should be disabled.