When/If my widgets refactor is merged back, the delete checkbox could be handled as a Widget I think, by treating it as a Field, just one which doesn't come from the form itself but is created ad-hoc by the FormStyleFactory.
I also just posted:
https://groups.google.com/g/py4web/c/b3uwN7G3aUI which proposes a refactor for the layer above widgets, namely how Forms handle the list of fields, which could, if implemented, make this even nicer by having a default delete FormInput instance.