Thank you for the clarification! It still looks a bit odd. For example if you provide both options, true and false, for the single Checkbox field, you will actually see two checkboxes on the form. And even can check them both.
And the value in you custom data becomes not boolean, but array of booleans.
Still it does what I need and even more. I have problems with Complete Registration form: it just crashes when the Self-consent field is the only field required to complete registration (will open the Issue on GitHub).
But now if you set the only one option (true) for the Checkbox field and make it required, it can nicely serve as a Consent field.