TYPO3 Core

fieldExplanationText works fine with Inspector-TextareaEditor and Inspector-TextEditor, but does not seem to work at all with Inspector-CheckboxEditor.
There might be more inspectors where it doesn't work.
Try this, which will add a couple of fields to both email finishers:

TYPO3:
  CMS:
    Form:
      mixins:
        formElementMixins:
          formEmailFinisherMixin:
            editors:
              2000:
                identifier: 'includeFormValues'
                templateName: 'Inspector-CheckboxEditor'
                label: 'Include form data in mail'
                propertyPath: 'options.includeFormValues'
                fieldExplanationText: 'The explanation text is not shown'
              2100:
                identifier: 'customText'
                templateName: 'Inspector-TextareaEditor'
                label: 'Alternative text for mail'
                propertyPath: 'options.customText'
                fieldExplanationText: 'This explanation does show'