Can’t believe there isn’t any explanation how to do this anywhere else… I’m putting it here as Google points here. So, for me this worked with Silverstripe 4.6 and UserForms 5.7:
SilverStripe\UserForms\Model\EditableFormField:
extensions:
- TractorCow\Fluent\Extension\FluentVersionedExtension
SilverStripe\UserForms\Model\EditableFormField\EditableOption:
extensions:
- TractorCow\Fluent\Extension\FluentVersionedExtension